Every three months, pick someone new to be in charge of getting everyone to cross-sell more. Doing this ensures that someone is always paying attention to what could be done to boost cross-sales.

And this doesn’t need to be a sales person; if fact there's a good chance that someone outside the sales team could do as good a job!

Also consider the benefits of giving them a little extra commission for all cross sales that the business makes during their period in charge.