I'm so jealous of you not having to wear foundation in the summer
! What's your secret?? It must be great not having to worry whether the foundation is going to be running down your face at some point during the day!
But to get back to your question, there are a few different things you could try.
Firstly instead of your normal moisturizer you could use a mattifying moisturizer (clean and clear, formula 10.0. 6. and dermalogica do moisturziers with a matte effect to name a few.). The key is to look out for oil free products.
There are also mattifying gels available, to help maintain a smooth, shine free look throughout the day.
You could also try using a mattifying primer on top of your everyday moisturizer to keep the shine at bay.
Lastly you could try using anti shine blotting papers and just blot your face whenever you feel it is getting too shiny. The papers usually come in a cute little boxes, handy for your handbag when you are on the go. The paper basically absorbs all the shininess from your skin leaving your skin looking matte.
Hope that helps.