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get yourself a 4/4 beat piece, then try to make that into a 3/4 beat. It should help and it should be fun as well.

I do something similar to what Ander said, only I use a 6/8 meter, since it can generally be transfered evenly into 3/4, and you can do little tweaks to it after the fact to make it a waltz (strong downbeat, bitone chords, and more).

Be aware about theses tips.
For my part, a 6/8 meter is different from a 3/4 meter.
I mean the "music sentence" (I'm french, sorry for the translation).

In a 6/8 meter, you have a "strong" beat on the 1/8 then a "weak" beat on the 4/8.
In the 3/4, you have a strong beat only on the 1/4.
With this, you'll have a melody that will be different.
It's like if you was speaking while walking or running. Even if the speech you'll say is the same, it will be different : rhythm, beats, phrasing...

When I'm playing (or composing) a waltz, I can do this:
the first measure : the root of the chord on the first beat, then the entire chord (with or without reverse notes) on the 2nd and 3rd beat.
The second measure: the fifth of the chord on the First beat, then then entire chord on the other beats.
Then I play chords of the song like this way.
After this, I can do some variations with the chords on the 2nd and 3rd beat (I keep swinging between the root and the fifth).

Listen to the 2nd Waltz from chostakovitch (or shostakovitch), you'll get a good example about what I mean :)

Hope I helped ;)

waltzes are my favorite type of music from video games...but when I try to make my own, I just end up reusing the same pattern - a I chord with the bottom note played first and the top two notes played two times to make a 3/4 tempo. For example, Waltz for the Moon. How can I make my 3/4 pieces sound more different drastically?
Hahah, as soon as I read "my favorite music from video games" I was like "WALTZ FOR THE MOON!"
It's one of my favorite songs ever, and one of the coolest parts of that game, imo. =D

I'd like to try writing a waltz too, so I'll be keeping an eye on this thread...