By offering  them some percentage of profit on every new 
sale.

As they are already achieving their targets, you need to
create a situation where they can outperform the set targets.
That means you need to give them a promotion, set a new
increased targets and new slab of commission.

In my opinion when ever the management sets target, they 
keep in mind the resources available to achieve those 
target. When we say resources, primarily it means manpower. 
Therefore if a sales team is already doing business & 
achieving target that means there is an optimum utilization 
of resources. Now if you want to motivate the team further 
then you should design a strategy where in you can 
communicate the companies growth potential to your 
subordinate vis-a-vis their possible growth oppurtunities 
in the organisation. You can always motivate them by 
telling them that more they add to the companies topline 
better are the chances of their growth in the company as 
what ever extra they are achieving is actually helping 
organisation to add to its bottom line & allowing them to 
afford more resources & therfore an oppurtunity for career 
advancement.

If a sales team is already doing business and achieving 
targets it means it has the caliber to do better and 
better.Scope to do better is always there.
So to motivate them firstly I myself will perform better 
than them and then I will set new increased targets for 
them and then I would make them believe that if I or anyone 
else can achieve that higher level target than they can 
also and I would offer some percentage of profit on 
achieving d increased target.

If the sales team perfoming better that means they have the 
enough caliber to perform better,i would only give the 
suggestions & motivate them to get the new markets where 
the older markets are running well,I give extra benefits 
provided for the new marketsale