bluffing with a set

[rikz]

I'd just check. A straight will call, but not bet. A higher set might call but not bet. A low flush will call but might not bet fearing a higher flush. A big flush will go all-in over the top of your bet and you'll probably have to call with whatever you have left just on principle ($100 more in a huge pot).

I think you just need to check it down and hope nobody has a 4, a 9, a higher set, or 2-spades.

I think the best line on the river is to check it down.