I'm currently in the process of designing and developing a WCF (web) service which is to be used over mobile phone clients(multiple platforms) for various things. The clients will need to authenticate themselves to the service and then regularly call the service to either update data or check to see if there is any data available for them to show to the client.

I've decided to developed a prototype project first to get acquainted properly with WCF before getting my actual project worked out. However, since I'm quite new to WCF, I'm not sure what I need to pay attention to.

The project I'll be developing:
- Should be able to handle several thousands of clients
- Should be able to be load-balanced

I was thinking about using a per-call with no synchronization (Concurrencymode.multiple), but all the information on concurrency and the various other configurable things is overwhelming. I don't want to develop something that needs a major overhaul later on because of bad decisions.

Any tips on this?

No great tips, since I don't have that much experience with them, but naively it seems like a lot of your problems are not with the WCF service itself. I mean that should be pretty thin. Provide a sane interface, send along to some data tier that deals with the synchronization issues. Those sorts of things are rather agnostic to WCF. Don't cache too much, transactions are good, normalization is good, logging and backups are supergood.