parts pay

We are trying to revamp our pay plans for our counter personnel. Currently we pay them a salary every week and at the end of the month, they receive a bonus based on the parts sales from each of the fixed departments. The percentage is based on the length of their employment and their performance. Our problem is, are gross is down this year due to our wholesale business and that beings their earnings down. We are trying to find a better pay plan that is beneficial to both the employee and us. Any ideas? Does anyone group their fixed operations together and pay a percentage of the total gross? Your help would be appreciated.