1) Offer fresh niger seedMake sure your niger seed (aka thistle or Nyjer) is fresh (less than 3 months old, whether it's in the feeder or in the bag). Also make sure the seed is loose and dry in the feeder.
2) Hang multiple finch feedersRedpolls are VERY gregarious. Flocks can number over 100. Having multiple finch feeders for them will allow you to host more redpolls.
3) Use a mesh finch feederMost finch tube-type feeders limit the number of birds on it at one time to the number of perches it has. A mesh finch feeder allows more redpolls to cling on the feeder to eat and the sight it amazing - it looks like a busy beehive.
4) Keep your feeders and feeding area cleanRedpolls are susceptible to salmonellosis, and because of their flocking behavior, they could easily spread it to others in the flock.
Help them out by cleaning your feeders often with a weak bleach solution (1 part bleach to 9 parts water) and by cleaning the area beneath the feeders as well.