Do you have mainly VAT registered clients? Or mainly non VAT registered? If you sell to VAT'ed clients, you add 20% on to your current fees, and they don't mind because they can claim it back.. you, on the other hand, get the benefit of reclaiming 20% on your purchases.. just a thought.
Then you want to delay as long as possible! Until you hit the VAT threshold (on a rolling 12 months), you keep all the figures gross, no VAT separation necessary. As Matt says, as you start approaching the threshold, you might want to keep a separate record doing a VAT split, to get a feel for how it will affect you.