Some styles mix well, while others don't. For instance, a stainless steel folding chair may clash with a Louis XIV gilt mirror. When in doubt, carry some snapshots to an interior designer and request a moment of advice.
Antiques, however, tend to blend almost anywhere. Even the starkest contemporary treatment can benefit from the addition of a rustic trunk or planter. It keeps things from looking too predictable.
Play with color. Virtually any shade, even orchid or lime, can now be considered a backdrop or neutral-so why shouldn't you add a royal blue love seat, a gunmetal coffee table, and a buttermint chenille throw if they're all things you adore? It's a safe, fun way to experiment. And that may be the best advice of all...have a good time!