Don't be his temporary tattoo. People only get temporary tattoos because they don't want the commitment of a permanent one. And temporary tattoos can be easily removed. So, if he only calls you to hang out when it's convenient for him, that's not fair to you. If you let him know that you're ok with being his backup plan, you'll always be the backup. You deserve to be his number one, and not plan B.
Redirect your focus. Try not to focus on the other girl. You just do you. Don't ask him about her. Don't act bothered when he mentions her. You're wonderful, so act like it! As far as you know, you're the only girl he's considering. Guys like girls who are confident, and even though it probably makes you feel super insecure that he is interested in someone else in addition to you, you have to pretend that you don't care.
Don't be desperate or needy. In the words of NE-YO, "There's something about the kinda women that want you but don't need you." Sure you may want him, but you don't NEED him. Don't always wait around for him to decide if he wants to hang out with you. Make other plans! If you do this, he will probably wonder who you're making plans with. And guess what? You don't have to tell him! He doesn't need to know that you were having having a spa night with your besties. For all he knows, you had a date.
Don't give him all of your attention. If he's not giving you all of his attention, then he doesn't deserve all of yours. One of my favorite quotes says, "Never make someone a priority when all you are to them is an option." There's always someone else you could be talking to, so work the room!
Call it a day. Know when to walk away from the table. If it's looking like he's insisting on stringing you along, back off. Afterall, you still want your dignity, right? Of course you do. The best negotiaters know when to walk away from the table. How do they do it? They don't get too attached to what they're negotiating for. If you need to wave the white flag and regroup, go ahead and do it. Don't feel defeated. It just probably wasn't meant to be. Don't get too emotionally involved. You have to make sure your emotions aren't ruling your life. Sure you want him to like you, but don't bet everything on it. If it works out, great! It doesn't work out, even better! Count it as a blessing. You don't want to be in the wrong relationship.