2CE is highly soluble and quite stable in water or ethanol. Personally, I like to dissolve it in Everclear (95% pure ethanol), but vodka works as well. You can use distilled water, but unless you add some alcohol you have to worry about mold and crap like that if you store it at room temp. for long.
I'm not sure what the saturation point at room temperature is for either water or ethanol, but I've heard of people going as high as 100mg/mL with no trouble. I think 10mg/mL is a good concentration - it's easy to measure out with a 1mg syringe or dropper to whatever dose you want, even if you use Everclear and take high doses it's not enough alcohol to get you buzzed, and all you need is an airtight container and you can store it in a drawer at room temperature for years without it spoiling or degrading in any way.
Sometimes, especially if you keep the solution in a fridge or freezer, the 2CE can precipitate out in small clumps or flakes. If this happens, just mix/stir it until everything dissolves again. You may or may not need to let it warm back up to room temperature to accomplish this.
