I've been experimenting a bit recently. I wasn't happy with the commercially available "rubbed effect" varnishes - the results were always a bit too matt for my tastes.
The process I've ended up with is (assuming the original finish is not flaking off) :
Sand with 320 grit
Clean thoroughly with some windex on a cloth
Wipe surface with tack cloth
Apply 2 thinned coats of good spar varnish
Rub with 4F pumice stone on a damp cloth
Rub with rottenstone
They sell the stuff at Amazon. Amazon.com: Pumice Stone, Fine (4F), 1 lb.: Automotive
With the pumice you have control over the gloss (I have some 2F for a more matt finish still. You can polish after the rottenstone with car buffing compound for more gloss).
This pic shows the final rubbed finish on the left, and the gloss finish after varnishing, but before rubbing, on the right :
Here's another tip : foam brushes
. Just use one for each coat. A lot of the bits of stuff that make your finish look less than great, come from re-used brushes
. You can get 25 of these for the price
of one decent bristle brush :