Be willing to loose what you invest and have a plan in case of failure up front. Don't manage your life thinking that your start up will succeed BUT once you start up operate like you will.
Do some research -- not just into the company but into the idea of starting up. Professionals who leave large companies are suddenly in the position of doing it all. Entrepreneurship is not for everyone and I recommend researching the idea of being an entrepreneur before doing it.
Take your vitamins, get adequate sleep, and be prepared for lots of setbacks and disappointments before you finally "crack the code" on how to sell your product or service. If software development is involved, be sure to find a competent tech person who wants to be your CTO so you have help figuring out what's feasible and within a the scope of the funding you can raise. Be sure to write your business plan early in the process, and keep refining it as you go. Someone else mentioned getting a mentor, it's a great suggestion, look to your nearest entrepreneur group for someone willing to help you get started. And get used to hearing "no" a lot until you work out the product/service offering and your sales tactics. Good luck!
Make sure that if you do not have all of the skills set to run a business, that you hire or outsource for the skills you are lacking so when you do launch, you have expertise in all areas.
Understand your unique value to a targeted base of customers. Understanding your competitive differentiation is imperative to developing your business plan, key objectives and revenue goals.
Work with someone, a mentor or business coach on a weekly basis before jumping ship! I am assuming a professional is someone in a current job. There is alot of groundwork that can be done, where you can invest an hour a day to your "startup" while not putting complete strain on your finances or stress on your body and mind. Its like building the bridge, creating something ready for you to step into. I am not talking about making it perfect but its often hard to focus on two things, hence having someone to bounce with a key to success!
Know what your capital needs are and double the figure - seriously. The road to success is lined with linked recoveries...
Do your prep work: 1) Survey's to get the feel of your customer base, 2) Choosing colors based on their psychology to your customer base, 3) Using all the web site tools available, and 4) Networking groups of all sizes and shapes.
Take time to plan carefully. Remember you are building a business not creating a job for yourself.
Develop a simple business plan, know your market and be clear as to how you are different from your competition.
You are probably full of good ideas. Start with the one that will be your fastest path to cash.
Get the support of a capable Virtual Assistant as soon as you're able! You can't do it all alone, and you'll quickly overwhelm yourself if you try. Get a VA who who can assist with those administrative tasks that need to be done so that you can focus on the tasks that are business-building.
I am not sure what you mean by professionals, but if it's someone working as a corporate employee, I would first advice them to carefully consider entrepreneurship. It can be a rewarding path, but it's hard work and not everyone would enjoy it. So they should make sure entrepreneurship is right for them. I would then tell them to get feedback on their idea and do some market research before launching into a new business.