Check out these "Micro Shirts"


www.outdoorsupplies.co.nz/swazi.1.htm#fabrics

It's the best first layer garment that I've used in a variety of conditions.
I am a heavy sweater and find that this fabric is really quick drying and blocks wind well enough to wear as a single layer on a warm day.
I have two of them and always wear them on every hunt.