I have been using the dry shampoo that sephora sells in the bins at the checkout for years and I looooove it! Smells great, is cheap, and totally makes your hair look and feel clean. It is by Oscar Blandi and it is like $10. I just bought a spare bottle but it is lasting so long I feel like I may never need to open another bottle! A little is all you need since you just get it on the roots and massage it in.
I just use it if I have not had time to shower and feel gross or if I did shower and did not want to wash my hair (to keep it from getting too dry). For the latter, I will just quickly blow dry the roots (because some water gets past the shower cap) and then apply it. You are not supposed to use it on wet hair. I am totally recommending it.
Also, if you are looking for other "freshen-uppers" for while you are trying not to wash your hair, I also use Aveda's refresher spray that comes in the light blue bottle. I bought the big bottle to refill my sprayer and that also has been lasting me forever! Between these two products no one will ever know when I don't wash my hair (except that maybe it smells even better than when I do wash my hair
). With the refresher I just spray it all around on top, underneath and in the middle but not to make it damp and limp, just as if you were febreezing your hair. Then I use the Oscar blandi pronto powder on the roots (which actually adds some volume too) and I'm good to go.
Just FYI, be careful if you are wearing black when you apply it. Check and make sure you don't have signs of powder sprinkles on your shoulder (or on your ears or anywhere else for that matter).