Some Tracking Tips That Will Make Your Mixes Sound More Professional

GAIN STAGING:
This rule is just about as old as the console, and still applies in our modern world of digital recording with or without one. Always keep your individual faders lower than the master fader. This also applies to pre-amps and anything going into your recording device. Having the pre cranked, and the final attenuator at 0 to not clip will most likely produce a bad result.
Recording tracks as close to 0db as possible:
In the world of 16 bit recording this was the golden rule. Now that 24 bit just about noiseless recording is the standard in every pro studio, and musicians bedroom this is not a good idea. In the 24 bit world this reduces the amount of headroom, and does not give you the room to move up the level without clipping in the mixing process. Give yourself a few db of padding when tracking, and the results will sound better.
COMPRESSION:
These days we use compression on just about everything. Using it wrong can be the death of some great sounds while tracking. Once it's printed to the track, you can't take that away, but you can always add more during the mixing process. If you are using compression while tracking, use it sparingly with ratios of 4:1 or lower.
Also remember that when recording electric guitar amps, the signal coming out of the speaker is already very heavily compressed. Adding an outboard compressor can sometimes make guitar tracks sound thin and lifeless.
HIGH PASS FILTERING:
Many modern pre-amps, signal processors, and interfaces have a high pass filter section built in. Using a high pass filter set at 80HZ on your tracks can greatly reduce the mysterious muddiness of your tracks, and add a good amount of headroom.
Don't use it on kick drums, bass, or any element with extremely low or thumpy frequencies. You will find this will help your tracks sit very nicely in the final mix.
DIRECT BOXES:
If you are recording keyboards, bass, or just about anything else direct into your console or interface, this can do wonders for the sound when used correctly. It's also a great tool for mixing amplifier and direct signals without an overdub. Most modern pre-amps have instrument inputs these days as well, so don't be afraid to use them. Chances are likely that the signal will sound better than plugging directly into your interface.