Do you draw them first, or plan them out in a 3D editor/autocad type of program? Do you ever go into forge and just start winging it? I have done a mix of both - some of my maps have been generally planned out. But the maps I like best (and especially the asymmetrical ones) tend to be improvised. I go into forge and put a few pieces together, or build a small base - and then I just wing it. I've had some really creative ideas come up from working that way, and the maps usually come together well, although sometimes I have to rebuild a large section when things don't fit together as I had hoped.
I usually come up with the basic idea for a map at some completely inappropriate time, but later I sketch it out on a piece of paper and work from there.
There are many ways to come up with a map. It starts with creativity, and then that can be expressed through different mediums. You can use Google Sketchup, draw it on paper, just start Forging with just an idea, or something else that I can't think of. Just Forge whatever way feels right to you.
Well...I don't really do maps...But I can give you some advice on a gametype. When I'm thinking of a new game, I try to think of something that's never been done before, and something that seems impossible to do. If you think outside the box, things will come to you a lot easier, and you can get down to working them into Halo 3's system. If I'm fresh out of new ideas, or my current ideas can't be applied, I look for inspiration. I'll look at some famous bad games, such as Omega Journey and Fat Kid, and wonder how that could actually work if done correctly. Or, I'll look at famous good games, and use those as a starting point for a whole new game. Point being, I always either strive to do something better, or create something new. Either way, the result is usually quite good in the end.
I look at architecture used anywhere. Campaigns from numerous games have great building structures and layouts. I try to use those and make Halo 3 Gameplay work with the structures.
Usually, I think of a great idea during school and quickly jot it down...after around 30 big improvements, I start building. Here's the first sketch of MLG Requiem. How it turned out: Obviously, I was thinking in 3D to imagine lines of sight and such, and the design got much better. If your not really able to do that, I'd recommend using Sketchup and the like. But I'd also recommend carrying a little notebook around with you. I carry one everywhere I go and sometimes random inspiration hits me and I jot it down and it goes from there.
I personally 2 one of 2 things everytime. I sometimes just have a thought and start placing things, but then i draw them on my sketch pad during class and come home and start forging. But i love more symetrical maps so its easy for me to make them. 2 maybe 4 bases that are exact on one aspect but with different ways of getting into them. if you download my upcoming map you will see what i mean =)