In this 1 week alone, I have seen 3/4 badly made maps.
So, here is a request to you to help us enjoy your map

1. Caulk, Caulk and more Caulk...
Use caulk EVERYWHERE the player cannot see. Infact, caulk everything and then texture things individually (a sure way of getting it correct).
http://codjumper.com/forums/viewtopic.php?t=1008
2. Mitres...
Mitre everything you can. If your whole map is made nicely, but your skybox is not properly mitred, then your map in my opinion is considered badly made.
Again same tutorial from above will help.
3. Detail...
Small brushes, and badly shaped (cut at odd angles) should be made detail (CTRL+Shift+D). Make everything that would not be considered a structure, into detail (so stairs and window frames = detail, walls and floor = structural).
http://codjumper.com/forums/viewtopic.php?t=1089
4. Snap to Grid and Grid Sizes.
Snap to grid must be on when mapping - always
Also, use the grid sizes. They will help you keep everything tidy, walls can be kept the same thickness without zooming in, etc.
Grid sizes are
1 - 1 unit
2 - 2 units
3 - 4 units
4 - 8 units
5 - 16 units
6 - 32 units
7 - 64 units
8 - 256 units
9 - 512 units
5. Clipper...
Use the clipper tool. Do not touch CSG functions EVER.
CSG Subtract - causes tiny brushes to appear which will stop your bsp from beaing created. Whats worse is trying to find it

CSG Merge - Not sure what errors it will cause, but the function is pointless if you are using caulk anyways.
CSG Holow - This is not a "quick and easy" skybox creation tool. It will also cause the map to create tiny brushes.
How to use the clipper tool is shown in the same tut as above.
(http://codjumper.com/forums/viewtopic.p ... ht=clipper)
6. Arbitrary Angles...
Do not use free rotation. Use the auto rotate functions (they rotate at 90 degrees). Or if you want 45 degree rotation, use abitrary rotation, but type 45 (do not click up until you reach 45).
7. Entities/Models...
If you have a lot of health packs and nades, when you only need 1. Entities such as health and ammo can be made to "respawn". Here's how:
Make the health pack or nade then click n to open the entities window.
At the top you will see a button next to respawn. Click it, then type in:
Key - wait
value - # (change # for seconds to wait to respawn)
This way, you have less models, which will help out those with lower fps.
8. Caulk as clips...
Basically, people keep getting advice to use caulk as a solid invisble wall.. Well, it works but is not correct.
Use the clip or playerclip.
They can be found in the common textures and are a red colour.
(common/clip and common/clipplayer)
There are probably more, but these are enough for the basics of mapping essentials.