I have 15 rats and they stay on my shoulder with just regular clothes. Are yours really young? If so, they will probably get better balance as they get older. You could wear a hoody as well. Mine like to ride in the hoods. Really young rats like pockets as well. The only issue I have is protecting my neck and chest and arms-I'm covered in a little scratches.
Mine stay more than fine on me even the two and half week old babies...I get up and they're all over me like I'm a jungle gym..being covered in baby rats is a very funny feeling...but I think cotton hoodies are the easiest clothing for them to hold on to