Regen fighting can be very powerful, although at the moment if you want to maximize your potential you'll often need to break out the calculator. (Note: you never actually need to do this, but it can significantly improve your potential.)
Calculate how much damage you deal to a monster with a single hit: your attack power (base damage * bonus damage, including any applicable bonuses like Warlord, Berserker, or Priest class bonuses) times the monster's resistance. (Actually the inverse of the resistance: to apply 25% resistance, multiply your attack power by 75%.)
Calculate how much damage the monster will deal against you with a single hit: the monster's attack power times your own resistance.
Now divide the damage the monster will take by the monster's level (times two if it has fast regeneration, in which case you probably shouldn't be regen fighting it anyway) to determine how many tiles it will take the monster to fully regenerate from the attack; then divide the damage you will take by your own level (times two if you're a Monk) to determine how many tiles it will take you to fully regenerate. As long as you require fewer tiles than the monster, you can kill it via regen fighting provided you have enough tiles.
(The calculations can become even more complex when you factor in your and the monster's staring HP values, your ability to cast spells and regenerate MP, and various other traits, but that's the basic approach. With experience you'll get better at estimating whether you can out-regenerate a monster or not.)
It's also helpful to become very familiar with the existing deities. You can often benefit quite a bit from piety farming - take advantage of an early Mystera or Binlor altar to gain piety by casting spells while you explore, then convert to a different deity once you hit 100 piety, resulting in 50 starting piety with deities who are often much stingier about what they reward. Binlor + BLUDTOPOWA is a very potent combination, since you'll end up with more than enough piety to use his resistance-stripping boon repeatedly. If you don't have any starting resistances, you can also desecrate his altar immediately after converting out; that plus a free glyph to convert can make it feasible to get GG's Enlightenment at a very low level, setting up the potential for massive bonus experience.
Finding out your deity selection is one of the most important things to do early on - making Scout Altar a very useful preparation in every dungeon other than the Tower. You generally want to refrain from picking up or converting glyphs until you know your deities, as some deities reward conversion with piety, and some deities have a preference for or against certain glyphs. Likewise, you often want to avoid killing too many monsters until you've planned out which deities you want to use - save magical enemies for Taurog, save undead for GG, save low level enemies for Tikki Tooki, kill undead before worshiping Dracul, etc.
One other, simpler but very important strategy: take advantage of spells that can be pre-cast - i.e., cast it and then regenerate the MP it cost before attacking a monster. CYDSTEPP, BICEPS, IMAWAL, ENDISWAL, and to an extent WEYTWUT and GETINDARE are all most effective if you pre-cast them.