I have a favorite well known brand of make-up that seems to have gotten thicker than I like, is there any thing you can add to it to thin it out?
Trial & error, depending what you prefer.
You could try thinning your foundation with primer, rosewater or fix+. I also heard of RCMA foundation thinner.
You could also go to Sephora and ask if they have any recommendations or find a good foundation that works for you. They also provide free samples for you to take home if you request to try it out before purchasing.
When I used foundation, I used a few drops of witch hazel to thin it. It work well for me. Nice bonus was it gave it a refreshing boost.
This is a common problem and is easily fixable without purchasing a 'thinner'.
I have never tried Pghgirl's suggestion using Witch Hazel but it sounds like a good thing to try.
My daughter and I usually use a few drops of our favorite moisturizer and this has always worked great for us.
You should try a couple of different ways to find what works best for you.