I use FL Studio 10 as well. I know the problem, trust me. Getting out a tune of 2 to 4 minutes can be really frustrating. Definitely if you don't want your tune to sound the same, all the way through. Variation is my biggest downfall as well.
Here's a little graph thing you can use though, on making Dubstep.
You can divide your song in parts. Keep in mind, every part is 16 bars/blocks. (on 140bpm)
[Intro] -You either go for an ambient sound, speech, or an already worked out melody. If you do so, try to use the melody that you're going to use after the drop.
However, don't give away too much, cause you still want to 'surprise' the listener. Use different instruments or VST's, then when you do after the drop.
[Build-up] - After the intro, you've got 16 bars of build-up. An easy way to do this, is to just copy the [intro] and add some 'rising' sound. Starts off low, ends high-pitched (usually).
At the end of the build up, add some kicks or snares that build-up in speed.
[A] - [A] is your main melody. 16 bars, whatever wobbles and crazy shit you want to use. This is also your main drop. Make sure to put some time into it.
- [B] is basically the same as [A], 16 bars of melody, but just slightly different. Add some tiny variation, to keep the listener interested. A crash for instance, or maybe an off-beat "whoop". Of course, you can add a second 'drop', but you don't have to.
[Build-up] - Copy your build-up, and simply place it after [B].
[A] - Main melody
[b] - Alternate melody. (Keep in mind, that there is no limitation to your variation. So if you want to create 2 completely different melodies, go right ahead.)
[Outro] - 16 bars of your tune either slowing down, fading away, or maybe some ambient sound, for nice effect.
With this, on 140 bpm, you'll have a nice lengthy tune and still keep the listener interested.
When making Dubstep, I always use this method. Now change up the drums and kicks and snares and claps and hats and cymbals and percussion's, whatever, here and there.
[Intro] - [Build-up] - [A] - [B] - [Build-up] - [A] - [B] - [Outro]
All with the duration of 16 bars.
As for Drum & Bass... that shit is to hard even for me to understand.
I can honestly say, that I've never had this much trouble, with a certain genre.
Making good Drum & Bass is truly an art form.
Good luck with it though!