It seems many of you are more accustomed to using Photoshop than Illustrator. Here are some tutorials on Lynda if you prefer Photoshop, or are just curious to see what you’re most comfortable with.
For the purpose of poster design, make sure to check out the sections in the series dealing with layers, luminescence and color, and retouching and healing.
For Intermediate users: http://www.lynda.com/Photoshop-tutorials/Photoshop-CC-One-One-Intermediate/124097-2.html
This intermediate series can also be very useful as it deals with creating and formatting text, drawing shapes, layer effects, among others. The first section also deals with automation to streamline your workflow. For you photographers, one section covers the tricky subject of exporting to high quality JPEGS for web publishing from RAW files.
These tutorials cover tools to manipulate fine details. These include using the Pen tool, Smart Objects, blurring to create depth of field, refine edge, among others… in general, details that can really take an image over the top.
And finally the Mastery series, for those above ‘advanced’:
Topics in this series are more specialized, and include lighting, shake-noise reduction, advanced layers, among others… they’re more focused on editing photographs.
I would recommend taking a good look at the Beginner tutorials and some sections of the Intermediate series, at the least. Specifically, sections that will be useful to you should cover layers, text, shapes, and topics on retouching. From the advanced series, the Pen tool can also be very helpful.