You can also try spooling the tape from end to end (i.e. fast forwarding to the end and then rewinding) before playing, to eliminate any tightness present in the tape that can give rise to some problems. But, as ibshaw has stated, the ultimate result will depend on the quality of the tape and the recording, and also the condition of the machine that you are playing the tape through.
If, however, you merely want to copy them in their current state in order to prevent further deterioration, you should be fine. Just spool the tape(s) as I described above to loosen them up first.
Having said that, it is possible to enhance the quality of the converted version, but this is time consuming and requires specialist software which is capable of editing individual picture frames that is usually only available to the entertainment industry.