Hi @Simon_Knudsen, thanks for reaching out! You can find some great tips in these threads:
- Consulting with External Clients in Asana
- Service based businesses.... preference of clients as projects or tasks as clients?
- Best Way to Setup Asana to Work Directly with Client
- Advise needed - introducing Asana to the team
I hope this helps! 
cc @pforumleader @ambforumleader in case anyone has more tips!