If you plan on harvesting continually, the plant[s] will need more fertilizer. fish emulsion provides the nitrogen component and a little phosphorous as well. Modest amounts every other week should suffice. Blood meal will also suffice.
Lemon balm is a late bloomer, it produces small white flowers later in the season than most other flowering herbs. When it is flowering, it is advisable to snip back the tips a tad to discourage flowering and encourage vigor. The flowers lend little to the aesthetic value and unless you plan on seed saving they are of no use.
You'll probably never get all the flowers pinched back, so you can still expect to find volunteers popping up in subsequent seasons. Adding mulch will lessen the odds of fallen seeds from germinating.
Harvest and StorageUse scissors or shears to snip off lemon balm leaves. The fresh leaves can be used for a tasty tea and as a seasoning for meats, poultry and fish. It is also incorporated into desserts and as a garnish for salads and sauces. It has reputed medicinal properties which are covered more extensively on the Lemon Balm Tea Page.
Lemon balm does not store well. The lemon scent lingers but the flavor dissipates when it is dried, so is best used fresh. If you want to have some in the winter consider incorporating it into Indoor Herb Gardens.
Thank You and Happy Gardening.