Anything which encourages the shoulder angle to open...
1) Lie on back with hands by thighs, sit up and lift arms above head, lie back down to start position - this can be done with some sort of resistance or with wrist weights on
2) Press up position planches - in press up position rock forwards so that shoulders go in front of hands, then pull back to press up position
3) Knees to handstand (start kneeling down, hands just in front of knees flat on the floor, lift bottom into the air (moving shoulders forwards over the hands) and raise feet to handstand.
4) Have the gymnasts cast 3 in a row on the bar. Catch each one at their highest point and tell the gymnasts how far they have to go to get to horizontal - You could even put them into horizontal from the position they reach on their own. It might not be as far as some of them think!
5) Straddle press to handstands
A lot of gymnasts don't realise how far they need to lean forwards to get to horizontal and/or are scared of it as the others have said, so try putting more mats under the bar or have them do it over a pit if you have one.