* You don't have to identify potions and scrolls right away, often you can keep them until you gather more information. For example, killed flies sometimes drop health potion and they are on early levels, so you can learn the color without using it.
* Before you get the Scroll Holder that protects the scrolls from fire, it's a good idea to stash all the scrolls in water at the beginning of each level and retrieve them back before going to the next one.
* Scrolls of Mapping are absolutely crucial on levels >20 because of the traps and they are relatively cheap in the first two shops.
* Summoning trap rooms are great to gather a lot of xp if you have potions of paralytic gas and fire/toxic gas. After the level is cleared, you just open the doors by throwing something at it, go as far away as possible, throw paralytic and then fire/toxic potions. And run to the next level.