I don't have any tips on making it easier to get their harness on, but I
just wanted to check what type you're using.
 
My first harness came in package with it on a paper cut-out of a ferret.
It was rounded nylon and had one metal adjustable clasp in the middle.
It was very tricky to get on, but unfortunately easy for Mikette to get
out of.  We were in my Mom's backyard and she was investigating a hole
around a root system when I pulled her out and got half a harness.
Luckily I got her out safely but later found out it's called a figure
8 harness ferrets are able to slip out.
 
The proper type is called a H harness.  It's flat nylon and has two
separate adjustable loops which are linked by one single nylon strip.
It's much easier to adjust and put on the ferret (should be snug, just
able to slip a finger underneath).
 
dooks
Risa
[Posted in FML issue 5216]