You may want to read this thread....it's 7 pages long.....but a lot of good information. I'd suggest long sleeves (but contrary to what I thought for years), definately not cotton. There's a number of really lightweight fabrics out there with UV protection and moisture wick away that'll make you feel like you're not wearing anything yet keep you cool and dry. I'd also suggest a full face helmet (again, contrary to what I'd thought for years), you're actually cooler in them because your head is out of the sun.
Get one with true flow through ventilation and tinted shield with UV protection.
Here's the link...... http://www.cvoharley.com/smf/index.php?topic=63586.0