Also, if the quest includes the halfling bloodmage that one is actually rather good (you can use health pots to fuel B2P).
Otherwise it's like the guys said, but there are catches:
Goblins - well, goblins can be player more for race than class. Save (or time) your conversions cunnigly and get full heals out of level ups (combine with fireball/pissorf and other mid/fight level ups). What ever you do don't convert like a madman when playing Goblins - convert like a madman at the best possible time (most likely during the boss fight). Also, dont pick up stuff you don't need - leave unwanted glyphs lying around and don't spend too much gold if you want to be able to buy & convert cheap junk with good conversion points from the shops.
Gnomes - well, it's pretty straight forward allthough there are 2 schools of thought here. Regardless: Take whatever you are playing and make him a spellcaster, alternatively focus on class (if say Berserker) and if the mana pots work for you, great, if not, oh well. Glowing guardian rewards ption conversio and punishes drinking potions so you can get some piety out of that.
School 1) Throw everything at the boss/bosses. Simply save all your conversion fodder for the boss fight and convert for potions then, dring potions and hit the boss with as many fireballs as you can. It's kind of like playing a goblin in a sense, but you only refresh mana and you always get one potion per cp cap pop.
School 2) Use the potions to level up like a champ. Being able to convert for burst mana lets you spike higher level monsters with fireballs. Since the game is tightly balanced, being able to throw one more fireball can actually be a big deal. Pro's aren't big fans of this since this is actually playing like a complete noob, but with potion races it's pretty much the whole point - you can waste potions (inteligently) and get away with it.
Halflings - Much like the gnome, but you generally need more potions to get a melee hit in. If health potions don't mean anything to your class, focus on the class and spend your potions to get a hit or two in anyway. The unique thing about health potions, though is that Dracul gives piety for converting them, TT has them as a requirement for a powerfull boon and Glowing Guardian also gives piety for them. You can also make your health worth a lot more if you have the appropriate resistance high enough (Monks, Paladins, Binlor, etc). If you have the Bloodtopowa glyph you can use the glyph to trun health into mana, and then heal with a potion, and get more mana (making halflings very good bloodmages). So it's back to 2 schools
School 1) Throw it all at the boss. Whatever class you're playing, save your conversions for the boss fight, convert stuff at appropriate times during the fight and get a bunch of free hits in. Getting your health, or even better damage resistance high enough let's you get more out of your health. With good enough physical damage you can really do a number on the boss.
School 2) Use them for overwhelming temporary advantage. Worship Dracul and dump cp into potions, then dump potions for piety and get yourself a few good boons early - what's not to like? Same with Glowing Guardian, get a temporary piety advantage, and focus on the class. Or simply use a few potions to kill stuff above your level (with potentially the IMAWAL bonus active for more XP) and reach the boss fight levels with a lot of popcorn to munch on.
Also: Sometimes a class really has no use for health or mana potions (rarely since anyone can attack and cast firebals). If you're 100% sure this is the case (and it very rarely is) just see if you can get the hoarder badge (or the parched badge for not drinking either potion). Always remember that Glowing Guardian and Dracul reward converting potions so if you can get use out of them it's still something. Also, the Pactmaker has a pact that gives you a little XP for some piety whenever you drink a potion even if the potion does nothing, it can also help. The Trisword and the Alchemist scroll also work off potion use (and something else does too, I think).
I almost got pwned by Shifty Brickwork!