If you don't have the service manual for your machine, you need to get one. Not only is it absolutely essential for proper maintenance of an analog deck, but these Tascam machines include lots of really helpful information in their manuals. Everything from a concise explanation of how deciBels work to detailed exploded diagrams of the machine's innards and exact calibration instructions. It's good stuff.
Keep your machine calibrated. Learn how to do it yourself, get the necessary equipment and alignment tape, and take the time to do it right. When you've mastered it, you'll have a much more intuitive grasp of signal flow, signal voltage vs decibels, frequency response, dynamic range, reference levels, etc. It will make you a better engineer. It will also allow you to predict what will come back off the tape at playback based on what you shoved in there during tracking. Without calibration, it's a crapshoot. Once you've done it a few times, you'll start to learn how often it's necessary on your machine (my 48 holds calibration almost indefinitely, while my JH24 goes to hell if I sneeze near it) and what sorts of things cause it to need recalibration (some decks get loony if you move them, all decks need calibrating if you remove the headstack, mess with the tape path, work on the electronics, or switch to a different brand or formulation of tape).
How to squeeze the most sound and quality out of a 15ips 1/2" 8-track. First of all, you have to be diligent about setting levels to maximize dynamic range. I tend to hit the tape HARD on certain tracks, printing a very hot signal to tape. The electronics on my Tascam 48 have tons of headroom (perhaps surprisingly) and have allowed me to do so.
The louder your signal is on tape, the less you have to "crank it up" in mixdown, which means less tape hiss in the final mix. If you're going to do any heavy compression, it's best to do it on the way to tape rather than coming off the tape. Compression to tape will help you get a hotter signal on tape, whereas compression on playback will raise the noise floor. This is common sense but it's helpful to point out.
This logic applies to EQ too. If you're going to cut the bass or boost the treble, do it on the way to tape. If you're going to boost the bass or cut the treble, do it at mixdown.
There are times when you need to push the VU meters WAY into the red, and it will sound fantastic. Other instuments will barely move the meters before you see the clip light flash and hear clipping. That's just the nature of different sounds. Short, staccato sounds (like snare drums, tambourines, castanets, wood blocks, etc) will be over and done before the VU meter starts to move.
I almost never bounce tracks. I have maybe done it two or three times in the 10 years I've spent recording on 1/2" 8-track machines. First of all, one of the major advantages of this format is that it forces you to conserve tracks, which translates into conserving sonic space. You wouldn't believe how many albums have been SAVED by not having room for that tambourine overdub, or 4th guitar part, or whatever. And being limited to 3 tracks for drums will help you learn very quickly where you really need to put a drum microphone. When we first got our 2" 16-track machine, the quality of my drum tracks tanked for about a year until I learned to NOT put up eleven drum mikes. You think you have what you need among those eleven tracks, but come mixdown you find your great snare sound is ruined by a harsh hi-hat and the good sounding hi hat has a weird tom overtone in it, and so forth. When you only have three drum tracks, you can pay a lot more attention to each of them and make damn sure they have what you need. Anyway, if you do decide to bounce tracks to make room for that one magical overdub, here's how you do it. You mix your two (or three, four, five, or six) old tracks as you see fit on your mixing console and you overdub that mix onto a new track that is NOT ADJACENT to one of the original tracks. There has to be an unused or saved track in between. If you don't, then you'll actually get magnetic feedback on the head and you'll record an ugly squeally tone. This is important to keep in mind if there's ANY CHANCE you'll have to bounce because if you fill up all the tracks with beautiful performances you can't lose, then you may be unable to do a bounce at all. Then you have to get the DAW involved and that's just no fun.

Anyway, that's what I know about tape decks.