This is a good question! I've written a post recently on this on my blog.
I have had this problem in past relationships, but I was scared & uncomfortable & really just not into it EVER. I was too scared to ever speak up. The person made it clear that he wasn't there for BOTH of our pleasure, but for his and strictly his only. How foolish I was!
I actually haven't ever really had that problem with my current partner. The only things I've ever wanted to change was maybe the pace such as, wanting something faster, slower, etc. Nothing big though. That's mostly because he knows me well & always wants to know what I like & dislike. He's very "giving" Lol So usually he asks what I like best and I've never had any situation with him where I didn't like it or wanted something different. That's something I'm grateful for, because I'm not good at speaking up.
But, the advice I gave in a blog post was maybe try letting him know the things you DO like by slightly making a noise (or not so slightly lol) If you don't want to come out and say it, that's a good idea for showing him.
I've also heard men say that they like for women to take over and tell them what they want. You could try that. Or you can say "do this or that" but that's not always easy.
You could find a good time and start up a conversation about favorite sexual stuff. That's a good way to tell him. Ask him to tell you what he likes in bed & in return you'll tell him. Then you're less likely to get him on the defensive side that way.
I sure hope these help.