If it is about the relationship, go somewhere where good things happened (say you went bowling on your first date, do that again) or go buy some symbol of the achievement; I use to take $10 to a large flea market/antique shops with hundreds of booths and have to find "the" item I wanted with just that amount. I still have my antique brass house key from the late nineteenth century that cost me $8 on my keyring to remind me of my secret library where I can get away and everything is wonderful in my head (think Secret Garden :-)

Start a new tradition. My husband and I have done that around Christmas (buying an ornament together, for example); if you've lost weight or something like that, reward yourself with a gym membership or go out and buy the same teeshirt for both of you or enter a 5k walk together, something meaningful to both of you and that you can look back on (and take pictures of :-) Take a picture and start a special photo album for successes such as this one?