THERE'S A BOOMLET of financial joy in Prince George's County, where residents are in for a hefty break on their property tax rate -- at least this time around. Even though many thoughtful budget-watchers are whispering "wait till next year," when the county may no longer be living off the fat of the state, the combination of big money from Annapolis and sophisticated fiscal surgery by the county is impressive so far.

First came a $14-million increase in state financing for the county. This helped County Executive Lawrence J. Hogan, whose budget recommendation last month included a proposal to reduce the tax rate by 30 cents. He also proposed across-the-board reductions in spending by most agencies. Now the county council has improved on this effort, but cutting still more from agencies that do not directly serve the public and eliminating some upper management positions and duplicated services. The result: another 5.5 cents off the tax rate, effective July 1.

The tax rates are not tax payments, of course, for the amounts property owners will pay also depend on their assessments. Still, both Mr. Hogan and the council have been able to reduce taxes even below the amounts required by TRIM, the charter amendment that limits tax collections to the 1978 level. So for now, county officials will let the good times roll -- and hope that the dollars keep flowing in from Annapolis next year. But experts in the capital have reason to doubt that Maryland will see another bonus like this year's -- which is why TRIM is going to have to be modified, and why current citizen efforts to prepare for this change should continue in earnest.