I too have some SNJ sitting at my desk, but I've never used it!! I do know that one of the tricks to getting a really shiny surface is to use a black enamel paint as an under-coat before the SNJ. If you also have the polishing powder, you can work this into the surface after it's dried and it will really shine up the paint. You can also work the powder directly into the black enamel and it will shine up really well too. Sorry I can't describe the technique in more detail, but perhaps someone else has the secrets...
I've also use small amounts of the polishing powder to highlight various areas of my models. Almost like dry-brushing, you get a small amount on the brush and then drag it over the raised details. I find that the best time to do this is when the paint has just dried (ie.not wet anymore) , but not cured yet (the paint will still be 'soft'). Be careful with this powder though because it really floats around in the air and I'm not really sure you want to be breathing this stuff in. I use a respirator/filter mask.
Good luck!
M.