Make sure you share deep down core values. Know that a happy, lasting marriage doesn't happen without the commitment and work of both parties. It won't always be easy, but if you are willing to work and not throw in the towel just because of some speedbumps along the way, you can have your best friend as your partner for the rest of your life.
Don't rush things. Don't skip the important step of slowly learning about each other through experience. A history with someone bonds you to them verses the instantaneously in love folks.
Don't ignore red flags. Expect each red flag to get worse and not better.
Look at his/her friends. Birds of feather flock together.
If the person doesn't have any friends, keep looking.
Listen to what your friends and family say about your partner.
If you don't feel good about yourself when you're with the person, keep moving.
Don't try to change someone into the person you would like to be with. You should accept who he/she is without the hopes and dreams that he/she will change and then decide if he/she is what you need/want in your life.
When someone shows you who he/she is...........BELIEVE it. You have NO power to change him/her.
For single people:
Never become involved with a man or woman who is committed to someone else. Don't expect a healthy long term relationship to develop from an adulterous relationship. In other words, if you are dating or seeing a married person, don't expect this relationship to flourish. It's best to stay away from a cheater altogether...
Look for someone who you feel is an honest person, and who shares your same principles and values...
^^^ I keep writing about marriage. If you are in a happy, long term relationship with your partner, regardless of marital status, share some tips on finding that special someone for those who are still looking!