o Sketch your maps before building them, it really saves time. You don't have to sketch the whole level out, but you need to have some early level layout. It is also a good idea to sketch some specific areas or rooms. Grid paper is great for sketching levels and trying out a few gameplay ideas.

o Study pro maps and try to include the things you liked in your level. Read pro Level Designer's (LD) interviews, most of the time they give some handy tips from their experience. Try to understand why pro LD's choose to design things in particular way.

o Start making a reference library, which later can be used for inspiration. Simply browse for images online and save them in the reference folder. It helps to have subfolders with names such as industrial, urban, etc.

o Beta test your maps, give them to your friends and listen to their feedback. Sometimes fresh set of eyes can spot something you've been too busy to see.

o Plan your maps carefully with vis blocking in mind. Like I said above, sketch and plan your maps, so you won't have to rebuild it. Most of the LD's mistakes can be avoided by sketching and planning your map carefully.

o Learn to "Think in 3D". Imagine what will look good in 3D and what not. Most people find it hard to see a map in their head, but the more you build the easier it gets. It's important to have this skill so you can visualize some parts of the level in your head.

o Make maps with different heights and interesting geometry. Good maps give the player a feeling that enemy could come from various entry points.

o With new-gen games you don't have to worry much about going over the poly limit. The biggest performance hit nowadays is lighting and dynamic shadows. Keep track of how many light sources hit the surface. Remember, even if the map is gorgeous looking, but runs slow, it won't get played (especially MP levels).

o Place weapons and items carefully and make your maps balanced. If you have powerful weapon in your map, then there will always be players in that area. Don't place weapon and too much ammo in the same place, make the player travel to other areas to pick it up.