If you are really poor on ammo, don't train trap skill, and try disarm arrow traps. You can also simply walk on them, but you will loose lot more HP.
Or as mentioned, use melee on weak enemies, flee when one to resistant comes (an early orc knight won't kill you, but if you en up using 30 arrows on him, with 25% chance of
mulch...).
Also centaurs are fairly good at throwing, so pumping some XP in and using spears, blowgun and javelins early/mid-game if you didn't find much arrows is viable.
And if you are VERY conservative, you can try to always have two or more enemies in you line of fire, so if the projectile miss the first, it will maybe get the second, dealing extra dmg per turn, but more than that, you will have to shoot one time less so you can conserve ammo. Another trick, if there is lava/deep water behind the enemy, instead of using "ff", use "f.", so if the projectile miss, it won't drop in.