Generally people agree hairspray and a hair dryer, but different products have different pros and cons, you're best off trying them yourself to see what you prefer.
to do what? spike it? liberty or fan? dye it? color or bleach? i take it you mean you have alot of fine hair (as in not course). to spike mine when it was long i used Knox gelatin when it was long. but there are lots of products to use that work great. it's more of a personal trying things out until you find what works best for you.
Thick/fine hair? Do you mean that the strands are fine but they are closely packed together or something? Anyway, here's what I use. Unfortunately, the hairspray/blowdryer method just doesn't do anything for me; I think it’s my hair type. So what I use is a combination of hair glue + hairspray. I run a little bit of hair glue (about a pea sized amount for me) through the section I am going to put up and then pull it up with a comb. The hair glue applies resistance to the comb so that you don't have to hold onto the hair; just the comb. I then spray the section with hairspray (got2be), blowdry, and move on to the next section. This method works fine for my thin hair and 4.5 inch 'hawk; I intend to keep using this method when it reaches a large size but I cannot confirm that it will work on a 7 1/2 inch mohawk. I find it to be a great method though if you have fine hair like me. I also discovered that the smell of the glue I use (ICE spiker) mixes nicely with the got2be freeze spray to create a really nice scent. Regardless of whether or not you use my method; if you use hairspray make sure to spray the hell out of the base and only mist the rest of it; when fanning out a Mohawk, less is more. I have also heard of people using egg yolks, glue (actual glue, not hair glue), and knox gelatine to put large Mohawks up; I’ve heard that the soap method stops working when your hair get longer, however.