I have been using the following solution for the last 6 months.
Instead of throwing that sandpaper out when the grit is smooth, shake it out, or rather slap it against the shop door or something to get the backside clean.
I then use the "cleaned backside" of the sandpaper to mix my epoxy on. I also use the cardboard of empty cereal boxes if I remember. 
At Walgreens I buy Q-tips by the thousand for $1.00.
I keep Q-tips on hand to clean up excess epoxy and other small stuff.
I also cut off the cotton tip to use to mix the epoxy. The only 2 part epoxies I use are JB Weld and the K&G 2 part (slow cure, it will be stronger) that you can add a colored dye to (powdered form). The rest of the 2 part adhesives I use don't need stirring.
I got tired of looking around the shop for stuff to mix a little epoxy, a nail here, a piece of wood over there.
These are 2 things I keep on hand anyway, so I use them, so now I wait before I throw that sandpaper out, making sure it was not something that I need a respirator to sand, throw those out IMMEDIATELY.
In the long run I will look for a stainless steel rod that I can hang from my workbench to mix epoxy, I'd rather not buy popsicle sticks. I can probably pick one up cheap, the price of scrap at Bataef's, the local Sanford and son goody yard (no, I can't call it a junkyard, they have a lot of treasures there). I did manage to pick up a few copper bars to use as bolsters in 3/16 and 1/4 inch thicknesses there.
) I ground the tip flat and use it for mixing epoxy, it works to knock out the occasional scale pin too. I like the little plastic clampacks the dremel bits come in, I slice the top off, use the backside, and use the cavity where the bit was too. I use crdboard alot too, the glossed stuff works best (cereal boxes like you mentioned) as it doesn't absorb the epoxy like corrugated cardboard does.
