Do: Make maps for players, not mappers. Sometimes they're one in the same, but most players are not mappers and do not have the same perspective that mappers do on the mechanics.
Do: Related to the above, get players / non-mappers to be the bulk of your playtesters. Obviously having other mappers look at what you do helps as you can get design feedback, but player feedback is the most important.
Do: Use GLaDOS and companion cubes where they fit. Including them too often or not enough are both equally problematic. Players love GLaDOS and cc's when appropriately used, timed, etc so give it to them.
Do: Iterate on your chambers. The first pass is virtually never good enough to ship with, but it might be good enough after some tests to determine whether or not it's worth iterating on. Proving the fundamentals are fun, then iterating when fundamentals are solid is a good step in the right direction to making a good map.
Don't: Don't make maps that include needless skill challenges. Most of the fun in Portal is the solving of the puzzle, not performing something tedious that requires finger smashing dexterity to do (read this as twitchy execution). This is especially true in Portal 2 since it's available on consoles.
Don't: Don't go overboard with metal textures. Having a lot of white portalable surfaces is a good thing when you can manage it. It helps not only visuals, but also allows for emergent gameplay and potential extra solutions that don't compromise the puzzle's integrity.