You can use a variety of software, provided by VMWare to do this. One of them best (IMO) is VMWare Consolidated Backup, altough it can only do file-level backup from Windows-machines. You can schedule it to run every x time.
Read more about it here (Page 30). It's for ESX(i) 3, but it hasn't changed all that much.
I, personally, don't backup virtual machines.
Don't get attached to them. When they fail, throw them away and build a new one. Make sure you have procedures in place that allow you to quickly set up a new VM and put the most recent backup of your data back. A great way to do this is to provision them. You can read all about it here (Page 17).
If you do backups, backup what matters. Backup the raw data. Files, Databases, ...