I do similar things to Glassman... It is a learned art really.
I will say, you should get in the habit of making disposeable mixing cups yourself that have very small measured lines. What you do is find clear plastic cups, measure one with a series of accurate measured lines. Your choice then is to measure cups with the FIRST cup you lined (your "master" if you will) inside the others and then use the ones you just measured and toss them... Or...
put the cup you want to use inside the cup you measured off. Pour your materials in the cup, using the lines on the cup on the outside (if this makes sense... it's better if you can picture it). This is a little technique I use, and all I waste are plastic cups.
I like to pour a lot of casts at once if I can... Like Glassman said, keeping molds handy that you can use a lot of isn't a bad idea. If you have like, say, the POTJ Imp. Officer head cast, keep it handy... It's generic, it fits a couple torso's, it can have the neckpeg shaped to fit a couple others... Good one to have extra casts of is what I'm getting at.
Or, if you want accessories, do a mold for a random small box or something... keep it handy, and if you have resin leavin's around, pour it into that mold. Voila, extra box for your displays. Can never have too many of those... Or I suppose you could but at least it's something.
Plain boxes usually don't give you de-airing problems either, so you don't have to tend that mold while whatever you are wanting casted in the first place may require your attention more focused.