One trick I learned from an older gardener/commercial flower grower: Give those annuals a "hair cut" once or twice during the season. To wit: Petunias - hanging basket: Take a pair of garden sissors and cut off about 1/3 of the new growth, both hanging down and projecting upwards. Then water. New growth will start at the leaf axils and presto! Poof! lots of new flowers. Works the same with petunias planted directly into the garden, give them a trim, not just deadheading, and lots of thickness and flowers will reward you.
Snapdragons - As soon as a flower stalk has mostly finished, cut it off just above the first or second pair of leaves below the spent flowers. New flower spikes will appear at the leaf axils and it will be thicker than ever. Ditto red salvia.

I second the "careful with fertilizer" note. Be careful to use a fert. designed for flowers/annuals, since you don't need to promote lush greenery, or longlasting woody stems. There are some fertilizers specific for use in flowerbeds, and some have a herbicide to reduce weed sprouting, if you need it as well. Thinking here of Pr**n & Gr**n, which does work well, I attest.

Enjoy those annuals!

DP