I have pretty aggressive tired on my truck and to get them clean I use Meguiar's Super Degreaser diluted 8:1. Coat the sidewall liberally, wait a minute, than scrub with a course bristle brush, rinse and let dry.
To simplify applying a tire product, I usually choose a spray like Meguiar's Hot Wheel tire spray. It works well and because it's a spray it goes on easily and gets into all those crevices.
Another method I've found that works well is a small brush. I take a product like Optimum Tire gel and brush it on. It takes longer but I like it better and it lasts longer too. Another option with Optimum Tire Gel is to thin 1:1 with water, pour it in a sprayer and spray it on.
There are a number of products available through Autogeek that work well.