This is what I do if I know something in the way of a roadtip is coming up:
Every time I break a 20 dollar bill, I stash at least $5 in a wooden box used just for that purpose. If you start two months or more before the anticipated trip, you can get a pretty good stash going. If that's too big a chunk, stash singles and quarters. Just gotta keep it in your head that you can not touch that money until the time of the trip.
Keep the calendar clear for the estimated time frame; if it doesn't pan out, you've got some free time and the cash to spend during it.
