When comes to miking toms I really do prefer to keep things simple. I use one microphone per tom. I know of folks who like to mic top and bottom, and doing so can definitely help with LF response, particularly on larger drums. But doing so also requires not only more mics, stands, cables, and preamps, it also dramatically increases the trickiness of getting proper balance between all the pieces of the kit, minimizing leakage, etc. I really do like to keep it simple. While I would argue that two mics on the snare and kick are so useful as to be almost necessary, my experience with double miking toms (and my experience mixing tracks that were engineered by someone else using two mics per tom) has taught me that the potential benefits of using two mics on each tom is rarely worth the hassle and expense (time is money!).
So a quick discussion of mics: I prefer a good cardioid dynamic mic for tom duties. Floor toms, with their greater LF content (compared to smaller toms) benefit from a dynamic with extended LF response. Most good quality dynamics will work fine, but the models more commonly used on toms include Sennheiser's MD421 and E604, Shure SM57, Beta 56 and Beta 57, Audio-Technica ATM23HE (for smaller toms) and ATM25 (a personal fav! See pics), Audix D2 and D4, and EV's N/D 408/468. I also enjoy BeyerDynamic's classic M88 on floor toms (great kick mic too!).
Condenser mics are also used, at times, for toms, but because of their increased sensitivity they are prone to greater leakage issues than a good dynamic, and there's also the chance that damage could occur via an errant drumstick, which most dynamic mics will easily shrug off without a second thought.
So let's start with rack toms (smaller toms): Begin by placing the mic about an inch or two off the top head with the tip of the mic just over the edge of the rim, pointed toward the center of the head. This is usually a good starting point for a well tuned drum providing a nice blend of attack and resonance. With the mic pointed at the center of the drum you'll get more focus on the fundamental with fewer overtones. If you want more overtones point the mic more toward the edge of the head.
If the sound has too much attack and not enough body, move the mic in closer to the head. Be careful not to confuse proximity effect (the artificially increased LF content produced by a closely placed cardioid mic) with the actual tone of the drum. You may desire lots of LF in the sound, but if you get the mic too close you'll likely discover, once the drum is placed in a mix with other instruments, that it comes across as all mud and no articulation.
Here are a couple of pics of an Audio-Technica ATM25 on a 12" rack tom.
Note the distance from the tip of the mic to the drum head. Also note that the mic is pointed toward the center of the drum. With this drum (single ply heads) this mic and placement provided a clear attack with defined, punchy tone that was fast and full.
I often see younger engineers positioning tom mics with the front almost touching the drum head. Doing so creates a muddy sound that needs to be filtered to fit the overall presentation of the drum set. Instead, let the drum breath a bit by backing the mic off an inch or two. The point here is not so much about isolating the drum (though I will be using editing and processing to do just that in an upcoming article), rather, the purpose of this placement is to capture the overall natural character of the drum to provide the tone and definition missing from the overhead perspective of that drum.
If you later decide to make the toms sound dry and in-your face, this approach will give you the ability to do that. If your placement is too close, however, and you later decide you want to lighten things up and make them more natural, you will likely find that harder to accomplish.
With floor toms (or larger toms) I recommend the same starting approach as I did for smaller toms, that is, the mic a couple of inches off the top head pointed at the center of the head with the tip of the mic directly above the rim. The only real difference between floor toms and smaller rack toms is that the off-center approach (with the mic pointed more toward the edge) is almost never flattering to a floor tom.
Larger toms resonate more than smaller toms do. Period. As a result, even with the mic pointed at the center of the drum you will likely capture all the sustain you will ever need. Pointing the mic toward the edge will, indeed, give you greater sustain with more overtones, but it will almost certainly be an overly resonant sound that will require editing and/or greater processing to bring under control.
I recommend you just point the mic at the center, and leave it.
You can still move the mic closer if you need more body, or further back if you desire to control it.
Here's an Audio-Technica ATM25 on a 16" floor tom:
This floor tom has plenty of resonance. Like the heads on the rack tom these were single ply, so there was plenty of sustain and tone to spare. You can see a strip of duct tape and a spot of moon gel on it to help contain the overtones a bit. The sound was big and full, with excellent articulation.
When you have the toms miked, have the drummer hit quarter notes going back and forth between (or around) all the toms at a tempo that let's you hear the majority of sustain on each drum. Listen for inconsistencies between each tom, that is, one may feel very short and punchy, with little or no 'bend' in the pitch, while others might have much longer decay and a strong bend to its pitch. The pitch bend issue can be addressed with tuning, but tweak mic placement as needed to get a fairly consistent attack-to-tone ratio from drum to drum. This makes all the toms similar in character, so that they sound like they belong together on the set. There can be musical reasons to not have the toms share a similar tonality, of course, but in general matching their tone through thoughtful mic selection and placement makes the drum sound more cohesive.
Once you're pleased with the sound, audition the toms again while bringing up the overheads to see if the image captured by the overheads compliments your close miked tom sound. If all is well, the overhead perspective will blend with the focus and definition of the close mics to create a natural soundscape that makes the perspective of the toms dimensional and complete.
That's about all there is to it with toms. Pretty straight forward stuff, but again, trust your ears.