I use PHD 2 to guide both on the slit or offset using a field star. The later version had some features added compared with the original PHD to help with spectroscopy guiding, particularly the ability to guide on a specific XY position which can be saved and recalled and to nudge the guide position a fraction of a pixel at a time. It is not perfect when guiding on the hamburger shaped split star image though as it has a tendency to hunt. Others swear by AstroArt which has a special algorithm for guiding on the slit and is being improved in other areas specifically for this application.
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=771
To find my targets (some of which are extremely faint when I am using the ALPY 200 – down to mag 17) I use a webcam mounted on the guidescope plus guider image to check and update if neccesary the alignment on nearby bright stars (using an EQ6 with eqmod and Cartes du Ciel) then zone in on the required field using the guider image. Astrometry. net usually solves the ALPY guider field for me if necessary but usually comparing the guider image with the projected DSS image in CdC gets me there. For me though spectroscopy was the driver that pushed me to a simple permanent setup. The setup and teardown time was just too time consuming.
http://www.threehillsobservatory.co.uk/astro/observatory/observatory.htm
Cheers
Robin