Apply an access-list to the inside interface. Say you only want to allow http, ftp and https outbound you would do something like the following.
access-list acl_inside permit tcp any any eq 80
access-list acl_inside permit tcp any any eq 443
access-list acl_inside permit tcp any any eq 21
access-group acl_inside in interface inside
I would review what traffic you want allowed outbound then apply the access-list.
When you want to make additions to your ACL down the road, say you forgot to allow DNS from your internal network, you then just add the new ACL. (You want to allow DNS in your initial access-list)
access-list acl_inside permit udp any any eq 53
