Realize from the start that no one--NO one--cares about your business as much as you do. It's your baby and ultimately your responsibility. Delegate, but stay involved.
Be very specific as to what you want done, when you want it done and how. Have as much on paper as possible. Policies & Procedures have been a lifesaver for me. They read the manual first, I train them, they refer to the manual later. The more detail you have for every step you want someone else to do, the better. Our P&P manuals not only have steps 1, 2, 3, etc., but computer screen shots, specific examples, timelines, schedules and checklists. And even with the best of written P&P, there will be errors. Resist the urge to re-do imperfections yourself, rather bring it to his/her attention and ask them to correct their mistakes.
Meet regularly with your manager. Have a standing weekly appointment, and keep a record of what you discuss. Any problems can be dealt with early if you are meeting regularly.
As for your clients, let them know you now have an assistant, but assure them that you are still available. It will be hard for some of your clients, but courtesy calls and emails to check in on them will be greatly appreciated. We email weekly reports to all of our clients and I have a "Memo" that goes with the reports. I also write an article that goes out with the monthly newsletter.Good luck!