I used to use a solvent- Park Tool Citrus Chainbrite- to clean the glue from the rims.
Then, I got to thinking- there has got to be an easier, less messy, way.
An electric drill with a wire brush attachment does an incredibly good job, and quickly, too.
Caution: wear heavy gloves, eye protection- if the spinning wire brush catches your hand it can hurt.
I clamp the wheel rim between styrofoam pieces in a vice- and just have at it. Very satisfying.
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Yes, rim tape is good and easy- but it is expensive.
It is not pleasant dealing with the rim cement, as has been noted.
But, just a half a tube, equally doled out on the clean rim, between the spoke/nipple eyelets, let it set for 10 minutes, then, starting at the valve stem pull the tire over the rim without getting glue on anything.
After the first time it is pretty easy.
A day ahead, before mounting tubular onto rim---Hint:
Warm the tubular tire in an oven for two minutes at 100 degrees Fahrenheit, then stretch it onto the rim a day before you want to mount it. Pump air into the tire, let sit overnight.
Have some cardboard under your work area to catch glue drips, saves trying to clean up a mess.