If you click middle mouse button to get to something other than the F7 hotbar, you can place as many spells as you like. It will put an X over the icon of whichever spells are not currently equipped.
You can then simply hit the proper number to equip the spell, then hit it again to begin casting. This also eliminates a lot of the annoyance of only having 3 spells available at a time, as you can access up to 30 spells with the f8-f10 bars. There are a couple of problems though:
1) It will not keep track of which spells have which boosters, and sometimes your boosters will get unassigned.
2) It will only move in one copy of the spell, so your stack of 5 fireballs or whatever will only be one fireball if you equip it with the appropriate number key. Since the game is so ridiculously easy anyway, I find I can live without stacks of spells and the convenience of quick switching between spells more than makes up for the lack of stacks imo.
The booster and stack problem are not always a huge problem, as the game USUALLY switches the spell in your left slot on your magic screen. Therefore, if you keep boosters and spellstacks in the middle or right icons, they generally stay okay. Unfortunately, I have had it switch them out on me occasionally.
1) Not true. They will be ALWAYS unassigned.
2) Not true. All the stack which has been assigned will be moved back.
Also, let me remind you:
BOTH the SLOT and the SPELL are REMEMBERED by the system. While you assign just the spell, it's position is remembered to avoid bugs with boosters, and unassigned (i.e. permanently boosted and stacked) slots on the amulet. The best strategy is to use 2 slots in amulet assigned permanently, and rotate the spells in the 3rd slot (boosterless, yet FULLY STACKED to the same degree as when you assigned them).