Graphics for Conservation is a great guide for producing graphics for papers and presentations. It covers graphs/charts and maps, with really practical advice. 

Here's a sneak detail from a more complex figure that's in press.
Underpainting using digital watercolour brushes is fun. Simply make your drawing layer "Multiply" mode, and use free digital watercolour brushes to paint on the layer underneath.
I think the best thing about this technique is that you can take an older drawing and jazz it up to go into a book or presentation (whereas you would have kept it black and white for the journal paper).

PhyloPic,org. I LOVE THIS SITE! Have you ever wanted to do something funky with lots of animal shapes? (well yeah, okay, I do). An amazing person has created this directory - by scientific / common names - of free silhouettes under creative commons licences. 
This week I made a draft logo with them, so that I could quickly test an idea I'd had. You could also create infographics really easily with these shapes; or add them to your graphs to help distinguish data series (e.g. frequency of moths vs butterflies, or to represents Birds and Mammals -- see example).
So far, the collection is at about 2000 and growing, and it's got a really nice range of plants and animals to get you going. Thank you so much, PhyloPic.

Conference posters don't have to be a neat set of text-boxes and graphs. Econnect have published a great article about conference posters. If you can, read their latest newsletter (March 2014) as it expands on the process of how they went about communicating their projects as posters.

What makes a good scientific illustration? by Trevor Weekes, The Conversation, February 2013