I mean, the answer is kind of circular. You spark interest by being interesting.
Some pitfalls I have seen happen regularly -
1) Performing too often. It's said that professional magicians perform the same tricks to many audiences, and amateur magicians perform many tricks to the same audiences. Related to this is the tendency for folks to use magic as a substitute for a personality.
Don't constantly perform for friends. They're clearly not wanting that. Sounds like your girlfriend is pretty much always up for it and that's awesome - perform for her. If people aren't showing interest, there's no reason to perform for them. Personally, I rarely perform for friends - A woman I met nearly a year ago, who lived with us for 5 months and with whom we have done multiple collaborations, saw me perform magic for the first time last Saturday. My general rule is, if I am feeling up to a casual performance, I will wait until people have asked me at least twice before doing anything.
2) Not scripting. Lots of people think they can improvise a script and do just fine. Very few people actually can. Mostly what you get from those "I like to improv" folks is, "Uh, uhm, so, here, you take a card. Ok, uh, I'll take your card - everyone remember this card - I'll take your card and put it in the middle of the deck, here. Like that. And then, here, you tap this little button here on the top of the deck, just pretend there's a button there, here, and look! Your card is back on top!"
No. Just no.
Script everything. A script, when properly learned, is one of the more sure fire ways to improve your performance and make it more interesting. It doesn't have to be a super wordy thing, either. Look at Eugene Burger's Gypsy Thread. Beautiful. Brahma and Vishnu in as few words as possible is how he describes it. Just give it a premise that people can relate to. Once you know a script well, you can go off script whenever you want to respond to people's comments or whatever, and then always have something to come back to.
3) Too many card tricks. Joshua Jay somewhat recently published and article about a study they did. In it they found that unless there were other things involved (Like a lemon for card-to-lemon) people generally lumped all the card tricks into one thing. They can't tell the difference, and when they think back, they just remember it all as pretty much one trick. So if someone mainly does card tricks, and mainly performs for their friends, their friends will probably remember it basically constantly repeating the same trick.
Get some variety in there.
4) No social skills. This goes back to my point above. Don't use magic as a substitute for a personality. Learn to hold a conversation without doing magic. Once you learn to be interesting without magic, being interesting with magic is easy.