The above is good advice. Participation is key. "Submissive" ≠ "lies there like a blow-up doll", which is a problem a lot of people seem to have -- they expect the dominant character to do all the work, and that's not at all fun.

Have your character give something back. The dominant character sets the tone, sure, but they are looking for something in return. "Getting to bone uke" is not a reward in and of itself. Have your character react, struggle, seduce, do things out of their own initiative, maybe trip my character up a little (how and if you do that depends on what kind of scene and character you're playing, obviously.) Be surprising and engaging. "She moaned at the xxx in her xxx because it felt so good" is not interesting. For example, if the dominant character is giving orders and expecting them to be followed -- don't just have your character go through the motions. You may want to describe what they're feeling as they follow (or don't follow!) -- are they disgusted/depressed/turned on despite themselves/eager to please? If they're completely submissive, do they try to actively think about pleasing the other character beyond what the order suggested? If so, how?

Tl;dr submissive is not the same as passive, and the latter is something to avoid.