(An easier method is to play the tape and press MONO on your amplifier. If you loose a lot more high frequency content, you're probably suffering from AZIMUTH problems.)
For small AZIMUTH errors, Tape Restore Live will not only repair the phasing problems, but also restore the high frequencies that are lost. This only works if the AZIMUTH error is relatively small, so use it for fine-tuning only.
How to fix the AZIMUTH on the cassette deck:
Usually the tape head contains a screw which can be used to adjust the AZIMUTH. Set your amplifier to MONO and listen using headphones. Then adjust the screw to get the loudest possible highs. (Note: For very course adjustments, don't set it to mono or you might not even notice the difference...).
Then, as a final step, use Tape Restore Live.
QUOTE (Dynamic @ Oct 1 2009, 20:36) 
The Tape Restore Live! plugin for Winamp claims to correct even for varying azimuth on the fly. I'd imagine it's best to adjust it on the tape deck first (or only on the deck), but this is another option and may indicate the extent of misalignment. Also, the same plugin approximates Dolby NR, hiss removal for mono recordings (given that the hiss is stereo while the desired audio isn't) and FM Radio Stereo hiss reduction. It might help you to determine whether improvement is viable and can be used in conjunction with Winamp's Disk Writer to output a processed file from your source file.
{edit: added link to plugin}
{edit: added link to plugin}