Listen to all of the tracks in your song with a critical ear. Be ruthless and decide which parts are essential to the effectiveness of the song and which are not. Just because you went to the trouble of recording a part doesn't mean it belongs in the mix. Focus on the parts that best serve the song and eliminate the rest and you'll get better results from your mixdown efforts.
Work on the stereo image by panning tracks. Resist the temptation to simply boost the levels of tracks that are not clear in the mix; their volume may not be the problem. It may be that their frequencies are meshing with other instruments in the same part of the stereo field. A gradual panning of the part in question is often all that is necesary to bring it out of the shadows.
Pan the drums to form a natural stereo image of the kit from the audience's perspective, with the kick and snare in the center, the hi-hat slightly to the right and the overhead mics panned hard left and right. Bring in the bass guitar and adjust the level until you have a tight-sounding rhythm section. Then you can add guitars and vocals to taste.
Equalize (EQ) your parts, picking the most essential ingredients of the song and working on those first. When it comes to EQ, less is definitely more. Rather than boosting frequencies to attain the desired clarity, cutting troublesome frequencies is usually a safer bet. Remember that the lower frequencies gobble up headroom, so cut them whenever possible. For example, low frequencies are more important to a bass guitar, so why leave them on guitar tracks where they can overlap and muddy the mix?
Add effects to your tracks, but do it sparingly. Like EQ, a little reverb, delay and compression go a long way. A good rule is to add the effect until it sounds good to you, and then roll it back a little. Overusing effects processors can take the life out of the mix very quickly. Reverbs tend to reduce the presence of a track, and compression squashes the dynamic range, so what initially sounds good may end up giving you ear fatigue in the long run.
Don't let the mixdown session run too long. When your ears tire after you've been listening for too long, it becomes counter-productive. You're better off burning a rough mix to a CD-R and taking a break. Spend a day or two listening to it on different systems, noting things that you don't like. When you go back to do a final mix, you'll have a better idea about what you want to achieve.
Onceyou're happy with the mix, it's time to master your collection of songs by taking these recordings with varying moods, styles and tonal character, and making them into a cohesive-sounding album. You'll do this by making sure that levels are consistent throughout and that the spacing between songs and the fades of songs feel natural and unhurried. You may also want to add a little compression to the overall mix to give it a little polish and sheen. There are many mastering software suites available to help you accomplish this. Two favorites of home recordists are "Izotope" and "T-Racks."