Once you've learned Rest Stroke Alternation, it is important to learn to alternate while changing strings.  This is easier said than done. 

Crossing strings can confuse the right hand into wondering what finger should play next. 
It will be simple if you just remember this:
THE FINGER IN THE AIR PLAYS NEXT

The following exercises will try to tempt you into repeating a finger, but if you just remember:
THE FINGER IN THE AIR PLAYS NEXT, then all will be well. 
Did I mention THE FINGER IN THE AIR PLAYS NEXT ?  Good Luck.

In order to assure that you are always moving to an adjacent string,
each of these exercises follows the same string pattern:

Begin on the first string,
move to the second,
move to the third,
and then back to the second.


Exercise 1) Begin with "i" and play four Rest Strokes per string
Exercise 2) Begin with "m" and play four Rest Strokes per string

Exercise 3) Begin with "i" and play three Rest Strokes per string
Exercise 4) Begin with "m" and play three Rest Strokes per string

Exercise 5) Begin with "i" and play two Rest Strokes per string
Exercise 6) Begin with "m" and play two Rest Strokes per string

Exercise 7) Begin with "i" and play one Rest Stroke per string
Exercise 8) Begin with "m" and play one Rest Stroke per string


The string pattern returns to the second string so, if you want to repeat the exercise, you will be on a string which is
adjacent to the first.  I would recommend repeating the last two exercises at least two or three times.  You would never use this right hand fingering in a realistic scenario, but it is excellent string crossing training.