I'm sure this answer has been given before, but what exactly needs to be done to get a prestige bumb?
Two years ago I went 18-10 with a 45 RPI and my prestige became b+.  Then I had a bad season, with a tough sos, and went 10-18 and a 130 RPI, and dropped to a b- prestige.  Then this past season, I went 16-12 with a 60 RPI, a 13-3 conference record, and a national tournament at-large bid. I was knocked out in the first round  Getting all this, my prestige didn't change.  I just don't see how my prestige can drop to B- with a 130 RPI, then remain at a b- after I get a national tournament birth. 

Prestige factors in your team's four most recent seasons, which the most recent of all getting the most weight.  You may have a had a good season drop off.  Keep making the NT and your prestige should improve, especially if you win some games once you get there.

wiins and losses are a factor.

That good Sweet 16 dropped off the radar, which kept you at a B-. 

Last 4 years you are averaging 17 wins, 12 losses a 1st rnd NT and an RPI of 69.  That's about a B-.  You need to win more games.  Average 20 wins, win NT games and get that RPI up into the top 30 or 40.  If Averaging 70 RPI was a B+, there would be no room for the truly great teams that are in the top 25 RPI and make the sweet 16 regularly.