I love Jane Fosters work, her use of simple shape combined with patterned line makes these screen prints work really well.
graphicdesighforum.com
I also really like Adrian Johnson's work it has a really retro feel about and I think this is the way I want to go in my designs.
I don't know who the artist is but again these have a retro feel about them.
Illustration by Jerry Smath
for "How do we know feature" in the children's digest magazine 1961. I like the use of simple shape and the limited colour palette. Glen Mullalys
photo steam on Flickr
is really good he has a collection of old magazine images which are great as a reference unfortunately he does let you download them.
These illustrations by Amanda Visell have a very retro feel about them, I also like the blocky shapes she uses.
These illustrations by Lab Partners have really captured the retro feel. I think the use of simple shapes combined with contrasting line work has helped to achieve this retro style.