The biggest mistake people often make is they go shopping without a list! In other words they have never done the introspective thinking to figure out what traits they want and need in a mate. There is also a group of people that have a "shopping list" but they lack the discipline to stick to it. If they are physically attracted to someone they'll ignore the "red flags" in order not to blow an opportunity to spend time with them. Each of us (chooses) our own friends, lovers, and spouse. Therefore we are responsible for our own happiness!
People also have a habit of bending over backwards to be on their best behavior when they first meet others. It's a cliché to say, "Be yourself" however most people choose to be the way they think the other person wants them to be. After there has been an emotional investment that is when they reveal their "authentic selves". It's not uncommon to hear someone say, "He or she is not the same person I fell in love with" We treat the (new) better than the (tried and true).
The next major mistake a lot of people make is they believe they can (change) someone into what they want them to become! The truth is everyone is looking to be loved and appreciated for who they are! If either person has to change their core being in order to make a relationship work then it means they are with the wrong person!
Another mistake is people forget that dating is supposed to be FUN! A lot of people actually hate the dating process. They have a negative attitude that screams "prove me wrong!" or they are trying to "fast forward" to see if he or she is "the one" on the first date!
(Your subconscious mind will never allow you to succeed at anything you despise.)
Another group gets so nervous about dating they become uncertain about what to wear or how to behave. Relax! It's just a date!
No one is asking you to decide on whether to cut the red wire or blue wire.