For the test project which code explained in the last post, I have been doing midi mock ups of guitar strumming patterns, matching up the velocity, speed, and range to the audio files to get a sense of humanization. From the analysis and past guitar playing experiences, here are few tips on guitar strumming.
- When strumming, most proficient guitarist work with subtractive method. Instead of counting where to strum, they have their hands moving up and down on the most active subdivisions and simply float over the strings without touching them when not strumming.
- Rhythm, groove, time-keeping! Always practise with a metronome no matter what. Simply do not rely on your own ability to keep a super steady beat, we are just too humane to do that. If your groove is right on, it doesn’t matter what you play. Play it confidently, and on beat, and you will sound more professional.
- Pay close attention to the musicality of your right hand, including dynamic, strumming speed, strumming range (which strings gets touched on each strum), and also the minor timing variations.
I will be writing about general musical rhythmical exercises later on, but for now, let me recommend some books to help you advance a bit in this field.
For starters, I believe it’s best to get hold of the basic concept of strumming, rather than learning bunch of patterns or licks, and this book is great explaination: Funk Guitar - The Essential Guide.
  
There’s also few websites such as Strum Patterns, or Justin Sandercoe that provides instructions on strumming and more.
If you are lazy enough, you can try this one: Rhythm Guitar Encyclopedia. This book is a massive list of possible rhythms suitable for different styles. You can simply flip through the pages and trying out different patterns.    