Every client assumes you are sitting at your PC (or Mac!) poised, just waiting for their job to come along. They assume you can do it in half the time quoted, and miraculously, it will be completed at a fraction of the cost you originally agreed. Every client knows they're far more important than every other, and every client is convinced you want nothing more than to dump what you're currently doing to meet their needs.
They are, of course, all completely.....right.
One of the first things you'll find out running a microbusiness is that work never arrives to fit in with your circumstances. When you need a new project, no-one will have anything for you. When you're full to the gunnels with projects, another will come along....and then another.....and then, probably, another.
There's no science to managing the workload (as the great Senator John Glenn, he of the Mercury 7 and the Space Shuttle once told me: "just pray for 48 hour days and 10 day weeks" (how's that for a name-drop)), but there is an art. It's all about managing expectations to ensure all your clients are aware of the art of the possible. To repeat an earlier tip: never over-promise. Let your clients know when you're busy but never put them off. Find out what their real priorities are - it'll enable you to prioritise your workload. And don't be afraid to push back on unreasonable deadlines or pressures to do more for less.
Think about what would happen internally: would a manager going to an internal contact expect an immediate response? Probably not. So why should they expect the world to be different dealing with an external supplier? Let them know what you can do - and then do it better than they could have expected. Then get on with juggling heaven and earth.