Just gently keep working at it on a daily basis and be patient.
We have 3 Degus (like a domesticated squirrel), and when we got them, they weren't very keen on even being touched. I offered my hand for them to sniff and nibble on everyday, and I can safely pick them up now.
If they're small enough, you can also lay your hand flat on their cage's floor, palm up, and let them climb on your hand. If they won't climb on, you can do a couple things:
Just keep working up to it, and be patient. First start letting them come to you, then lightly pet them. I've found that most rodents don't like their head or surrounding areas touched until they trust you, so focus around their front legs and surrounding area so they can see what is touching them.
Try putting a treat in the middle of your palm, and lift them up gently when they climb onto your hand to grab it. Note, this might scare them, and may erode trust. It's a necessary step though, so just be careful when/if you do it. If you're going to forcibly pick them up, don't do it too quickly, and if you do get them up, hold them over the tank for safety, and don't hold them for too long in the beginning. Your goal here should be to show them that you can support them without problem and build their trust.
It's all about trust, so just do what you can to get them to trust and know you. If one day they're abnormally irritable, back off a bit and let them be. You don't want to lose any trust you've gained by forcing the matter.