Source: Thomas Lang

Steve Smith recently turned me on to the following “staggered doubles” exercise:

RRLL RRLL RRLL RRLL
RLLR RLLR RLLR RLLR
LLRR LLRR LLRR LLRR
LRRL LRRL LRRL LRRL

You can also add a check pattern consisting of a bar of singles, like so:

RRLL RRLL RRLL RRLL
RLRL RLRL RLRL RLRL
RLLR RLLR RLLR RLLR
LRLR LRLR LRLR LRLR
LLRR LLRR LLRR LLRR
LRLR LRLR LRLR LRLR
LRRL LRRL LRRL LRRL
RLRL RLRL RLRL RLRL

Repeat many times. A great warm-up.