This is your Merit sending graph with 5 days per column:

I'd say: "Don't spend them all at once"

If you spend your source Merits evenly during the first 30 days, they'll also replenish regularly, so it'll never take long to get a few new ones.
Anyway, props to you for running out of sMerits--that means you're probably doing a good job as a merit source. I need to step it up myself.
Easy: just give more per post, theymos prefers when Merit sources empty their source:
It's best if sources try to exhaust their source allocations, even if it means giving posts higher amounts than is typical.