Keep an open line of communication. Talk to each other. Never go to bed angry or upset with each other. Work things out before you go to bed. Allow one another to grow as individuals and be genuinely interested in each other.
Some people just go a little bonkers when it comes to love.
For years she's asked students in her class on interpersonal communications to interview friends about the most romantic thing that's ever been done for them, and that they've done for someone else. Probably most important is that the gesture is thoughtful, rather than particularly grand or expensive. Mental and emotional effort count. For instance, one boyfriend bought his girlfriend an entire outfit, down to the pantyhose and shoes, which she considered a tremendous effort on his part. One young woman put together a scavenger hunt trail of clues to lead her boyfriend to a Valentine.