Me and my husband both own our own businesses.
My top tips would be
Do your research. Don't just jump in getting all over excited about your idea. Make sure you're offering something people want. So often, people start up with something that nobody actually wants and they plug along with it. Talk to people. hell, survey monkey is free, design a survey and start emailing / fb it. It doesn't have to be something unique, it can be a loaf of bread as long as it's a loaf of bread like nobody's ever seen before 
Don't sell to yourself. It's what other people think that's important. Give people what they want, not what you think they want.
Accept that you aren't going to make any bloody money at first!
. Make sure you can live without any income for a year (just in case!)
Decide up front if you're going to be a sole trader, limited company etc. Look carefully into which is right for you.
DON'T take on staff until you HAVE to! Don't take on extra costs until you HAVE to! So many people get carried away and want the office and the receptionist and all that show. But it's a crippler if you do it before you're ready.
CASH FLOW FORECAST! Get one done. eat, drink and breathe the bugger. If you've not got the figures for a full 2 years ahead - what's wrong with you! 
network. It's a poncy concept
but it's got to be done. Get out there. Get chatting. Make contacts. Use social media - fb, twitter, linkedin
Don't leave things until the last minute. Don't bung your receipts in a box and sort it out at the end of the year! Be professional, be organised.
Think carefully about your business's image. That name, that logo, that tagline - they're more important than you think!
Don't get personal about it. Some people are really overprotective and act like any criticism of their business is a personal criticism of themselves. You have to get over that and learn to take it! Accept it. Examine it. If you get angry or defensive then you're not cut out for it!
Look at yourself. Do you actually, honestly, seriously have what it takes? It is HARD to start up and to succeed. Long hours, little money. No sick pay, no annual leave and everything falls on you. If you are in your heart, an employee - you'll never make it!