The material on my pack is too thick to get a needle through. If I melt the goo on the back of one of my patches and stick it on, will it stay? How do you get them on your pack and stay?

not worth the effort - leave the patches at home and have a lighter pack

Use Aleene's fabric glue, it is available at Walmart in the craft section.
Get the washable kind.
Worked for me.

Find a local boy scout store and get some patch magic or order the stuff on line. It's the ticket for your application. I have patches on my scout uniform that have been washed a dozen times and it shows no sign of letting loose. On the other hand with some goo-gone, WD-40 or dry cleaning the patches will come off. How cool is that? :banana

http://www.scoutstuff.org/BSASupply/ItemDetail.aspx?ctlg=05NDC&ctgy=PRODUCTS&c2=UNIFORMS&c3=UN_MISC&c4=&lv=3&item=501BMK

Use a needle from a sewing machine or from a shoe repair shop and a pair of needlenose pliers to push it through. I have sewn a couple of thick patches onto my Lowe Alpine packs made with heavy cordura material with no issues, I then seal the seams and teh thread with seam sealer.

Super Glue For Fabrics;)

Gorilla glue (use less than you think you need - it expands as it dries) or decorative patches. If you want to sew them on, you need to go buy a set of upholstery needles (not very expensive and have lots of uses for gear-making).

gorilla glue