Here's a tip I borrowed indirectly from our colleagues in the U/S guided biopsy
field.  The cap of a pen (obviously without a pocket clip), when pressed firmly
but gently into the skin for a count of about 5 seconds will leave a distinct
mark which will last for several minutes.  I map the vein as if I had a marker
pen then wipe off the gel and connect the dots with a marker pen.  You can devise your own system
for marking the location and direction of branches.  I usually double check
branch location after the main conduit is marked.  I indicate the size of the
vein at the groin, knee, ankle and foot.  If you are having trouble visualizing
veins try sitting the patient up on the edge of the elevated bed with their 
foot resting on your seat between your legs.  It sounds kinky but it works and
it beats getting all bent out of shape crawling around on the floor.  In this
position you can comfortably scan the LSV and the calf portion of the GSV.  
Mapping the LSV in this position takes practice and you may wind up back on the floor. 
Always remember that with the patient sitting or standing, the hydrostatic
pressure on the veins approaches that of an artery so they don't compress all
too well, don't be fooled into concluding that they are noncompressible.  If in
doubt, retest with patient supine.  I hope this has been useful.  We do a fair
wack of vein mapping and it seems to work.  We get lots of feedback from the
surgeons on our accuracy and we are rated for QA:  a score of 3 means bang on;
2 means close but no cigar and 1 means next time put your glasses on. 
Fortunately we have had mostly 3's.

Steve Knight RVT
Fletcher-Allen Health Care
In affiliation with the University of Vermont