I teach on lesson on this in the Asana Training Masterclass. My approach is to use individual projects to manage clients. That way you can potentially even invite them into that project as a place to get information from them and ask questions and vice versa. It also helps them know what's being accomplished and when. I think this works well for Virtual Assistants especially.
In some cases, this might be TMI and so you could have a second project for more of your team's tasks. I think the biggest factors for how you set up are how many clients you're regularly serving at one time and how long the engagements last.