I'd say I'm not that good at it either. I think there are two general approaches you can try:
1) If you are artistically inclined, you can start from the visual end of things, and just try to put together a level that looks good. Then tweak the skillset (and terrain if needed) and see if you can come up with a decent puzzle as well. Starting with all skills available in generous amounts, and then gradually see how the level plays out with less and less skills (particularly if you take away certain types of skills altogether) is a good way to proceed.
2) If you are more left-brained, you can try to come up with puzzle ideas based on various tricks and techniques you might have seen from official levels as well as other people's levels. There are a number of (by no means exhaustive) general ideas you can look at:
a) various means of crowd control, beyond the simplest of blocking to hold them off
b) multitasking
c) see if you can make the release rate a core part of the puzzle
d) take advantage of transient effects of performing a skill. That is, instead of simply looking at the effects of a completed bridge, or a through-and-through basher/miner/digger tunnel, think about how a lemming may take advantage of a still-in-progress bridge or tunnel. As a simple example, a lemming can turn around once at the wall of a digger's tunnel just a little bit before the digger breaks through, and you would have diverted a lemming to go in the opposite direction than where everyone else would. Some skills like bashers can even be interrupted mid-stroke to create interesting effects on the terrain (in the case of basher, you can create a sort of "step" that way).
e) Look at skills in terms of the exact area of terrain added or removed on each "stroke/step" can reveal some creative ways to make use of them, beyond their standard uses. The "A.T." trick is one spectacular example, where we consider the fact that you can create a single step by digging down the sides of a wall almost but not quite to the bottom, and then multiplying that effect to create multiple steps to scale great heights. Less spectacular but other useful concepts:
- a miner's stroke not only takes out terrain below and in front, but also some terrain overhead. So if you have a thin platform overhead you can break it in addition to the standard mining-tunnel effect.
- digging near to a thin vertical wall will effectively break through the wall.
- think of a builder's bricks as simply general added pixels of terrain, and you can come up with many interesting uses beyond the standard bridge. For example, stack a few build bridges to create a sort of wall. Fill up tiny notches/gaps on a wall with a build brick to make the wall climbable. Use bridges to provide terrain connecting two walls, so you can mine through both with one miner. The possibilities are endless!
f) Sometimes you may want to use a skill not so much for its "typical" effect, but for some secondary side effect (and if you can take advantage of both effects at once, good for you). A classic example is to turn a lemming around by having him build and bump his head on a ceiling. Even bashing on thin air, which seems completely useless, has the side effect of delaying the lemming, which may well have significant consequences down the line.
g) definitely make good use of climbers and floaters. They are the means by which a selected number of lemmings can get to places normal lemmings can't
Perhaps the ultimate advice is simply practice, practice, practice. Start creating levels without worrying too much about how bad they might be starting off, and one day you may find yourself becoming quite a decent level designer! Besides, there are many level solvers out there of different calibers, so you don't have to make it a goal for every level to be the hardest puzzle ever. Just aim for fun, enjoyable, interesting.
