A good source for this would be to look at the documentation for PROC PLAN, Example 68.7 Crossover Designs. Down towards the bottom, it presents an analysis using PROC ORTHOREG, that includes carryover (sequence) effects, that specifically depend on the previous treatment. Note that the analysis depends on using the EFFECT statement, so if you are not on a version of SAS/STAT that supports this statement (9.3 and 12.1 do), this may not be possible to run.
In any case, the model statement should give you what you need to get started.
Steve Denham