I was going to do ten tips, but I very quickly got to 10 and thought, who wants' to be limited to 10, its so 1990's. Nonetheless I have stayed with tradition and kept it at 10. Some are general, some specific. Tips 5 to 7 could literally save you hundreds of hours, because thats what they cost me. So here goes....
General
1. Modelling very much follows the 80/20 rule. The last 20% takes at least 80% of your time, and sometimes it feels like it's closer to 95/5
2. Starting to learn modelling with Blender is like giving a hand grenade to a baby, the results are seldom pleasant. Since 2.5 the GUI of Blender is better now, but it is still dangerous in inexperienced hands.
3. Check every step of your tool chain to make sure it works, and I mean it really works. Import/Export can be complex, difficult and error prone, often it depends on legacy 3rd party tools, that may not work with your exact version. So check, check and check.
4. Read the Cube Modelling Guide http://sauerbraten.org/docs/models.html, then you will find you will have to reread it frequently if you are making progress. It is your best friend.
Player models in Red eclipse & mekArcade
5. No spaces in your bone names. If you do the model will distort with animations, but will load OK as a static model. Applies to smd and md5. Haven't 'tested' with iqm. This one cost me a lot of time.
6. Keep player mesh names unique. This refers to the mesh name used within the mesh file not the mesh file itself. If not then problems with animations of one model may affect them all.
7. MD3 are unsuitable for complex player models, performance starts to die once you get into hundreds of frames. Use a skeletal format and harness the GPU
8. MD5 versions number shall always be 10, no greater, no lesser. If your exporter puts out a different number, make it 10
When dealing with artists in a commercial fashion
9. Don't pay them out until the job is properly done.
10. Make sure you get the source model, even if it is in a format you can't handle properly. Get all the animations and exports.
Number 11. This is possibly the most important. If your model is not working as expected, reboot your system. The Cube2 engine does no housework with the trash, so dud model data stays in memory and can affect your models behavoir.