One tip is to look at how other big models (whether they're sets or MOCs) are built and try to reverse-engineer parts of them that you like. You can learn a lot of useful techniques that way, and that can give you a starting point so you can then start mixing in techniques of your own. I spent several months reverse-engineering a MOC Christoffer Raundahl had on display at LEGO World CPH 2013, and then spent many more months changing and refining things until I had come up with an original MOC.

I also find LEGO Digital Designer useful, because sometimes when you are building big, you won't have all the parts you need on hand right away. Building ONLY on LDD can be limiting, but I find it can sometimes be useful to alternate between building part of a model physically, then using LDD to record what you've built. Then you can borrow parts from what you've already built without worrying that you won't be able to put it back together again.

Sometimes a good way to come up with ideas for big models is to experiment with larger parts. For instance, you can take a few big armor pieces and try to find a way that they fit together nicely. Then, try and find the simplest structure you can come up with to get them to attach together in that position.

It's never a bad idea to approach a larger model in sections. Sometimes it might take a long time to find a way to connect those sections together into a cohesive whole. Sometimes they won't end up going together at all, and the foot and leg you come up with will end up going on different models entirely. But each time you come up with something that you're proud of, or see a technique in a set or MOC that you admire, you can add that to your repertoire for the future, and then it's simply a matter of finding which builds or techniques from that repertoire go best together.

[AFOL voice] superglue

*pours huge amounts of parts on desk*

*starts building*