Donna- Winter-sowing is an excellent way to do "succession sowing" as you describe. You'll have plenty of plants to replace those that peeter out this time of year, and your gardens will look MAH-velous all season.

I'm in Z5, and 'sorta' did a little SS by sowing some Cosmos in early July and tossing them in an area that desperately needed them. Additionally, I put in Morning Glories late, too, and they're just coming into bloom now...again, I'm in Z5, so you're about 4 weeks ahead of us.

But head over to the Winter Sowing forum if you've never been there...it's WAY too easy!

Winter sowing isn't worth the bother here in sunny CA-
but I do agree with successive plantings.
''Cosmic Red'' Cosmos just bloom like crazy,and I've always
got tons all over....
Cleome is another plant you can use,for it seeds all around here,and I sow them in intermittent batches.
Try Portulaca for some late,but brilliant color.
And Gomphrena takes the heat of late summer well.
Use Zinnias like nothing else-the ''Profusion'' series are some of the prettiest flowers you could grow!