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Since I speak from experience, I'm going to answer this one.
1) Don't even think about it unless you LOVE what you're doing. You are going to live, breath, and bleed that business for as long as you have it, so if you don't go in from the start with a keen, fierce passion, there is no point.
2) Have enough start-up capital to run your business without a single sale for the first year. Even if you plan for 100 "what-if" scenarios, there's always the possibility of that 101st coming along, and you need to be prepared for all contingencies. I was actually in the process of opening my first retail store on Sept. 10, 2001...you literally never know what you're going to be facing until it hits you.
3) Know your market. Do your research, and know exactly who you're selling to. Where is your money coming from, and why are people going to give it to you instead of to someone else? Over and over again I hear people thinking about starting a business say "It's perfect, because there is no competition!" I say if you think there is no competition for your product/service, it simply means you haven't looked hard enough. No business idea is 100% unique, chances are you're just putting together existing ideas in a different order or location. There is competition, and if you don't see it before it sees you, you're wasting a huge opportunity. If you don't see any, ask yourself "why not?" Look particularly to the big corporations. They have money at their disposal to do in-depth research, and if they're not in your neighborhood or field, find out why--perhaps they know there's not actually a market for their service, yet. If you've truly tapped a new, growing market, still, keep watching the big guys, and keep in your business plan the "what will I do if/when Big Competitor X starts offering this service/moves in my area?" question. Keep in mind that as a small independent, there is always, always somebody out there with more financial power than you, and in the early years, you're going to have to compete by virtue of being nimble and quick in comparison. I'm not saying you can't take them on and win, I'm just saying you have to set the stage for your victory and know in advance how you're playing the game. Think David and Goliath, baby: David and Goliath.
And best of luck to you.