Insuring and adjusting the Vacuum can result in smother shifts. However, another reason for shift problems is wear.
On some models of transmissions the Reverse Band in the transmission is adjustable from the outside (but there is a Shear Bolt you need to remove) on others you have to take apart the Transmission.
Good article on adjusting you transmission; but be patient as you have to read about 1/3 of the way through before you get to the interesting part:
http://www.continentalimports.com/ser_ic20242.html