First try the high contrast filters. You can even use a No. 47B (C5) filter to get the most from the paper. High contrast paper often doesn't give the best results, however. If you are still unhappy, try selenium toner (pretty strong, Barnbaum recommends 1+1 dilution for 15 to 20 minutes for full effect) on a negative you are not too attached to. I've seen this do wonders for flat negatives.