Congratulations on gaining your practice licence!
I think everyone will say that different things have worked for them; for myself most of my clients have found me via my website and a couple have come through recommendations from a local accountant.
When initailly looking for clients I tried several things from placing business cards in newsagents and local businesses windows around my home town in Devon, placing a newspaper ad, getting on Yell and other business directories via a free listing (be wary as many will then try to sell you a 'premium' space, be firm!); social media and writing to all the accountants in my area. I also attended a starting in business networking event which allowed me to gain a self assessment client.
I don't there is any tried and tested method other than doing all you can to ensure everyone knows about you - send a press release to your local paper, hand out flyers but whatever you do, make sure you have a good website in place using something like Wordpress - if you need any pointers, don't hesitate to PM me!