There's one thing every list needs, above and beyond all the usual stuff that we talk about. More than anti-tank, more than suppression fire. More than mobility, or anti-infantry, or bubblewrap. More than counter-assault or deep strike defense; more than even scoring units or anything else, lists always seem to need more points.
I'm going to tell you how to the rarest of tricks, namely how to get those points (without cheating.)
Scrounging for points is nothing new- every list builder ends up doing it sooner or later, and usually more of the first than the second. Most codices (sorry, Necrons) have a lot of cool toys in them and it's hard to fit everything you want in, but at some point it comes down to brass tacks (or, as the accountant above might tell you, down to terrible brass thrones atop an infinite plain of war) and you need to start making cuts to your list so you can fit it into whatever point level you're playing at. This is hard, because there are always some really useful units that you want to keep in and upgrades you just can't live without. But you chop away and finally pare the list down to... oh what the hell, you're still 100pts over? This is just not fair, there's nothing else you can cut from the list without ruining it!
Calm down, friend, I'm here to teach you where to get those last hundred points, and probably from places you won't even miss them. Learning to artfully snip away the dead weight of a list isn't easy, but it's something that comes with time and practice.
Tip #1: Always start bare-bones.
The easiest mistake in the world to make in list-writing goes something like this: "Okay, I'll start with two Tactical squads; they each get Multimelta, Meltagun, Rhino, Extra Armor, Power Fist, and Combi-Flamer. We'll add in a Librarian, and since he's gonna go with the Termies he get Terminator Armor and a Storm Shield." Or, to take another example. "Alright, we need an HQ and I want to run a lot of monstrous creatures, so we'll grab a Hive Tyrant, give him Old Adversary so my other guys are good in combat, Hive Commander since I want to be able to reserve, a Heavy Venom Cannon for a little extra suppression, Armored Carapace so I can soak missile hits, Regen to make him tough with Leech Essence, and a pair of Tyrant Guard with Lash Whips." And you very quickly find that you don't have nearly as many points as you thought you did.
The lesson here is to start squads at the bare minimum they need to function and nothing else. Once you're further along in the list-building process, you may end up with some spare points- that is when you look back and say "what other upgrades can my squads use?" But in the end, more bodies is usually better than fancy toys. To quote a friend of mine, "Usually, the best upgrade you can buy a trooper is another guy to stand next to him." So start with the very least you can get away with- in the above examples, that is the ten Tacticals + Rhino or the Tyrant, Old Adversary, and his Guard. "But you could cut more points from both of those units!" some of you might be thinking. Yes, but their basic roles are defined by these upgrades- the Tacticals need a heavy weapon and a bunker to sit in, and the Tyrant needs protection so his support abilities function. (In a different list, you might consider Hive Commander the shoe-in, but presumably a walking Tyrant isn't focused on reserves.)
Tip #2: Lead yourself not into temptation; keep units focused.
Another easy mistake is also visible in the above example: the generalist's fallacy. Every unit in your list should have a role; many will have several, but tend most strongly towards a particular one. Our Tactical Marines are there to score a point and add a little shooting in the process. The Hive Tyrant wants to hand out buffs and turn himself (and the rest of the army) into a major melee threat. Before anything else, all your upgrades should focus on making a unit better at its role.
Let me repeat that again because it's important: every upgrade you give a unit should make it better at its main job.
This is the biggest trap I see people fall into. "Well, upgrade X would make me tougher, and upgrade Y would give me an option to do this other thing poorly..." No. Don't do it. Why does that Tactical Squad need a Power Fist? They shouldn't be in close combat in the first place, and if they are, you want them out of it as soon as possible so you can shoot the enemy or charge them with something like TH/SS Termies. Why did you give the Tyranid Warriors Deathspitters? They should be running forward every turn to get into a fight, not hanging back and looking for targets. What good is a Storm Bolter doing on your tank with AP2/3 weapons? Do you WANT your opponent to be able to abuse wound allocation? Take what a unit needs to do its job and do it well, and nothing else.
Tip #3: Don't throw good points after bad.
Sometimes, a thing is unsalvagable, not in the sense that it's worthless, but it has one facet which simply can't be helped. Imperial Guardsmen and Dark Eldar are both ridiculously fragile- buying them survivability upgrades is usually a pretty huge waste of points. Orks virtually always strike last in close combat- grenades are not going to change that. Usually upgrades that attempt to cover a glaring weakness are going to be expensive and somewhat mediocre; do you really think paying 30pts for a 4+ save is going to change the fact that your Company Command Squad is five guys with T3? No, your opponent is going to aim some Bolters their way and end them unceremoniously, no matter what you give them.
This comes up other places, too. Giving a squad a single combi-weapon doesn't make them any good at a role, and it can encourage you to do dangerous things with them under the idea that "they can probably kill that tank, right?" Having options is a good thing- giving yourself bad decisions to make is not.
Tip #4: Focus, Focus, Focus.Trying to fit too many different gimmicks into a list- even good gimmicks- is another common cause of problems. A 1500pt Space Wolves army is simply not going to have room for three squads of Long Fangs AND some Speeders AND TWC AND Lone Wolves AND a big HQ or three. Think very carefully about what you want your list to do, and what it realistically can do, and choose your units appropriately. If you are building an assault-heavy list, think twice before putting in a static unit with a Lascannon- do I need this fire support, or do I need more threats? Flexibility is all well and good- indeed, it's necessary, or you end up with a shoddy rock list that only has one plan and crumbles when it fails- but the flip side of that is focus, which keeps your whole list working on a single goal. If you have fire support, make sure it is going to be the kind that deals with the targets you need to worry about. If you have countercharge, make sure that it can be where you need it and handle the targets that are going to get to you. Always insure that all the parts of your list are aimed in the same direction and not just randomly doing their own thing.
Tip#5: It's okay to give up sometimes.
Sometimes you can't find room for everything you need. Rather often, in fact, especially if you are trying something new. Never be afraid to admit defeat and start over from scratch- the worst thing you can do when writing a list is get completely hooked on one build and pound away at it for hour after hour, tweaking and twisting and modifying until you have a pile of gibberish units and no focus for the army whatsoever. Before you EVER play a list, set it down for a while and walk away. (I usually find a day or so is sufficient to distance myself from it, but you may need less or more time.) Then come back, sit down, and re-evaluate it from scratch as though you were reading it for the first time as someone else's creation. How does it fit together? What is it doing? What is it missing? I find this little bit of separation is often enough to give me a new insight into things, showing me gaps and oddities that I hadn't noticed before. This will very often be the stage where you have to say "fudge it" and delete the whole thing- perhaps last night the nine-Biovore, six-Lictor list looked awesome, but in the morning light you may see it for the unsalvagable garbage that it actually is.
This is completely fine. Everyone makes bad lists. I have deleted literally hundreds of lists of every point value without so much as a second thought for any of them. Most of them were quite awful. The sooner you can get over the fact that most of the lists you write are going to be bad, the sooner you can start writing good lists- the secret to creativity is knowing what to throw out and what to keep, when all is said and done. Moreover, the more lists you write, the better you're going to get at realizing what is trash, what isn't, and when you can change things.