Lee--
It's not necessary to practice frequently if you've reached a level you are happy with. But it's always a good idea to refresh your skills every now and again (in fact, you might find that sporadic practice periods may actually help develop new insights and skills, so long as the practice is not TOO spaced out -- perhaps two to four weeks apart, on a sort of "as you feel in the mood" kind of schedule. Of course, the ideal "practice" is when you have real targets to attack (anything from dowsing a sports outcome that you have money on to dowsing the location of a missing child).
I think it would be interesting to get input from folks who have had success with practical dowsing, whether it is something relatively small (such as finding missing keys or -- like I did once for my brother -- finding missing sprinkler system parts lost in his basement). That would encourage others, and give those who post such experiences some braggin' rights!
Best,
Paul