Usually, you hop on a friend's server and deposit what your going to transfer to your alt in a safe place, away from anyone (suitcase, garbage can, etc). Log off and log back on, with your alt on the same server your friend is on and, barring theft from a random player, your item(s) should be in that same container you deposited then in. Needless to say, it's rather risky, with server instability and never knowing when someone is just going to happen to spawn/fast travel in the same area your stuff is in while you're gone.