One of the most consistently good pieces of advice I know of for nearly anything is: Know what you want.
I recommend making a list of different qualities you want in a partner and separating them into categories: Musts, Maybes, Soft Limits, Hard Limits
Musts: These are things a perspective partner must have for you even to consider them. Look back on past relationships if you can and see if you can find things you never want to deal with again and things you never want to be without. These are big things and I'd recommend focusing on personality or behavior traits, not physical ones here. An example of one of mine is: Emotionally secure/high self-esteem.
Maybes: This category is for all those things you'd like in a partner. Think Alanis Morissette' song, "21 things I want in a lover." High Intelligence is one of mine here.
Soft Limits: This is the inverse of maybes, for everything you'd rather avoid in a partner. Don't like people younger than you usually? throw it here. Remember some of these may cover up other preferences. Ex. Avoiding younger people may mean you want someone more mature than you are. I have smoking in this category.
Hard Limits: Last, hard limits is for those traits you simply will not put up with. Three of mine are codependency, abusiveness and manipulativeness. I've been lucky to have avoided all three of these behaviors from my girlfriends.
Once you have completed your first list, keep it in mind. If a friend tells you something his or her partner did that you really liked, consider putting that trait on your list. Try and have as few Musts and Hard Limits as you can.
Some suggested traits to think about: Emotional security, self esteem, codependance, abusiveness, manipulativeness, jealousy, romantic, outgoing, introverted, large body part, small body part, Not in an exclusive relationship with someone else, interested in you.
Once you know what you want (more or less), you can start thinking about the person you're interested in a new way. As long as you keep this as mostly general guidelines and don't go too far with rating other people you'll be much better off. If you start carrying around a clipboard and a pen and rating different people on paper in front of them, this is a sign you're going too far.
From there, determine the kind of relationship you want to have. If you're waiting for marriage to have sex, don't go to your local pansexual group looking for potential Mr/Ms Rights. There are all sorts of relationship configurations, even though most people reading this probably are looking to be part of a heterosexual couple and live happily ever after. Could you be in an open relationship? Could the person you're thinking about? They're definitely not for everyone. Do some research online or read a book like "The Ethical Slut."
The best time to define or redefine a relationship is at the beginning. The sooner you decide "I want an open relationship" (and figure out what that means to you), Ã‚Â the easier it will be to bring up with your partner. This can be as simple as, "I'm not dating seriously right now" or "I believe in love polygons."
The important part is your partner should have a chance to find out what you want out of a relationship so he or she can decide if they want the same thing as you. You may be a bisexual woman looking to get in a multiple gender relationship and run into a straight guy who wants to be dating several women. Make sure both you and the other person are on the same page as far as what you want out of the relationship goes. It saves lifetimes of hassle when done well.
Turning knowing what you want into getting what you want means being willing to not accept less than what you really want. You may also find you need to get over some of your issues before the person you want wants you. This is natural and normal.
It may seem like a lot of work, but trust me when I say (as a guy whose done it), it is worth every minute of work.