I will offer my vast experience to the dilemma (okay, maybe half-vast) ...
First the DO NOT'S ...
Do not buy appliances, even if that's what they say they want. And really, you shouldn't have to be told that. If they want a blender or a dust buster, by all means buy it for them, but not for Valentine's day.
Do not buy crap at the grocery store on the way home. How romantic is cheap candy, an over handled stuffed animal and half wilted flowers?
And, unless the stuffed animal has special and personal meaning, skip those too.
Jewelry is dangerous. One, unless you are willing to spend significant dollars (or Euro, drachmas, Rubles or whatever your local currency) that sub-$100 prefab heart shaped schlock says "unimaginative cheapskate". If you do buy decent jewelry you set up a dangerous precedent for the following year. Be careful.
Do not take them to Denny's for dinner, even if that's all you can afford. A romantic TV dinner at home (if done right) is better. And if you do go out, make damn sure you have reservations. If the restaurant doesn't take reservations, pass.
Lingerie ... now there's a conundrum. If you go "Frederick's of Hollywood" you better be sure she's into the the porn star or stripper look, or the only thing you'll be getting is frustrated. Victoria's secret could be more romantic, but any type of provocative clothing implies that there will be further activities, so make sure "further activities" are mutually desired. And for all that's good and Holy ... MAKE SURE you buy the right size. Too small says "I'm too fat" and too large says "you think I'm this fat?"
Gift cards ... don't. Just don't.
Finally, don't use Valentine's day to try and fix a shaky relationship. Fix the problem, don't bury it in candy-heart sayings. This isn't third grade. (unless it is for you, and then you should definitely NOT use the lingerie idea)
Now the DO's
Be unique and personal. Make the damn card even if it's a finger painted library paste monstrosity. It shows thought and effort.
Candles=Yes. TV=No. Theater=Good. Sports=Walking home.
Dress nicely, even if she isn't. You know you don't care what she's wearing as long as it comes off easily, but she cares that you look good. By the way, she'll still probably look better than you so don't hesitate to tell her so ... multiple times ... sincerely. Fake it if necessary.
If you do the flowers and candy thing, buy a small amount of decent candy. It doesn't have to be in a heart-shaped box, but it should be something she likes. And get flowers from a florist at least two days prior to Valentine's day and keep them watered in a cool dark place so they don't become mulch before you give them to her.
Use your sense of humor, but make it cute. If you want to be provocative buy a small canister of helium and make her a bunch of balloons out of multicolored condoms (unless you are picking her up at her parent's house, then that's probably not a good idea).
Most importantly ... be confidant. Women can smell fear and apprehension on you like a German Sheppard and take it to mean that you are either a dork, or you don't care.
Men everywhere have feared February like a beast in the wilderness ever since Saint Valentine was shot in the buttock with one of Cupid's cute little heart-shaped arrows (I may be a bit off with my history).
