I keep an old cookie tin in my desk drawer. When I don't burn a candle to completion for whatever reason, I put the stubs in the tin. Then, on my once-a-year camping trip, I use the candles to start the campfire. It's ecological (I'm reusing) and it's a sound practice spiritually.
As I throw the candles in, I think of what I've used them for. Dinner with friends? I thank the Universe that I have friends and that I can afford to share food with them. Healing intention? I thank the Universe for those who have been healed, and send good thoughts to those who could not or would not be healed. If you're not spiritually inclined, consider the other effects of a simple moment of gratitude for whatever good moments the candles represent.
I also have a shoebox where I put candle holders that need cleaning. Once a year, or when I run out of usable candle holders, I clean them out. I usually try to schedule it for months when it's warm enough to go outside to do it or at least to dump the hot water I used to dissolve the candle wax. (See the Resources section at the bottom of the page for links to cleaning candle wax. Two suggestions -- freezing candles first and putting a little water in the holder before you insert the candle -- might even cut down on the need to clean.)
By the way, if you're fighting clutter, you should be extra careful of the fire hazard of burning candles. I'm fortunate enough to have a fireplace, so if I just want to burn a candle to scent the house, I burn it in the fireplace.
If I want to be able to see the candle and enjoy its ambiance, I put it on a fireproof surface somewhere where cats, dogs, kids, and strong winds (and curtains) from an open window can't reach it. Make sure your surface is very clutter-free first, and that, my friend is another motivation for decluttering!
(P.S. If your mind is as cluttered as my desk, set an alarm or kitchen timer to remind you to extinguish the candle.)
ResourcesThis link has good information on cleaning candle holders, reusing candles and wax, and other candle-related topics.
How to Remove Candle Wax from Glass Containers
This site discusses removing candle wax from other items. I'm going to try the Dawn dishwashing liquid after I do the nearly boiling water bit (I don't trust boiling water not to break the glass) to see if I can make the process quicker.
How to Remove Candle Wax from Just About Everything
Did you know that frozen candles burn twice as long and drip less? Check this out, only if to see her cool candle holders!
Fun with Freezing Candles