The following are some important and useful mapping tips and techniques i've learned during my time at TWHL, and through my own experimenting. Please feel free to share some of your own.
10. Start small and compile often. When I first started mapping, I made this huge thing complete without compiling once. Needless to say, it wouldn't even compile, and was so messed, i deleted it and started againe. Complete a couple things, compile, try the map, and repeat. Building a map in smaller, more managable chunks has saved me much time and misery.
9. Number nine sort of goes together with 10. Build all your basic architecture first--avoid all entities, doors, lights, etc at the beginning stages. Get the r_speeds, views and layout in order, and then, start adding entities. This will save you compile time and head trauma.
8. Research. A short time spent on research saves hours of time building, and then rebuilding something. Even If you think you're sure about the details of what your planning to do, take 2 minutes first and google for some reference pics. I definitely learned this lesson the hard way...
7. Use Vertex Manuipulation, not Carving. I mapped for almost 6 months before I even tried to use the VM tool. I just used cubes and wedges for everything. Don't fear the VM, but one suggestion. After you VM a complicated shape, hit 'ALT-P' to toggle the 'check for problems dialog'. This will save time, and help teach you what you can and can't do with VM.
6. For big complicated, or 'busy' maps, use Visigroups and the Cordon tool. One of the first things I do when starting a new map is hit the little red 'hide selected objects tab, to hide the outside walls of my map. this makes it easier to focus on what i'm working on at the moment, instead of things peripheral. Cordon tool saves much compile time on huge maps. Don't leave home without it!
5. Take your time and brush things as precisely as possible. Do not allow brushes to intersect. When it's impossible to make a clean connection between two brushes--like some weird angle stuff--leave the edge a unit away. To facilitate this, zoom to the smallest grid size and carefully line them up. Evil practices like Carving routinely created brushes that are off grid. Stay on the Grid! Trust me on this.
4. Use CTRL-M to rotate brushes, and check with 'Alt-P' after rotating complex shapes or groups of them. Don't use the clicky thing to rotate. Also, after rotatin, hit the VM tab and line up those corners on the grid. Keep it clean.
3. Hold CTRL and click to select multiple brushes or de-select one. Use CTRL+g, u, w, t to group, ungroup, change entity back to world, and tie entity to brush respectively. Click on a brush and hit ALT+ENTER to view it's entity properties. Click a brush while holding shift key, drag the brush somplace else and unclick to quickly copy a brush. I use these constantly.
2.Take time to read tutorials and posts to expand your mapping know-how, and try not to fear the unknown. You will save time and minimize misery, if you take a little time to learn about new things, rather than agonize over doing them willy-nilly later. You can almost always find the answer to a problem by googling/experimenting. Asking lame questions that you can easily right yourself is just unecessary. If you've spent some time trying to right something, but just can't, then post a question.
1. Try to help others when you can. If someone posts a question that seems obvious, try to remember that it isn't obvious to everyone, and nobody knows everything. Try to remember the time when you started mapping, and how arcane some aspects of the editor and the engine were, when you were a nebby. If a post makes you angry, then simply don't reply to it. Posts made in anger are always regretted.
Happy Mapping!