By 'untilled' do you mean this is soil that has just been weed killed or from which all the weed growth has already been stripped?
At the very least you would need to rake over the bare soil so seed sown has a chance of germinating.
There are quite a few 'green manures' which would grow well during the summer months to keep weeds at bay. I particularly favour phacelia as it seems to suppress weeds and also produces wonderful flowers which attracts lots of insects. It's easy to remove when you want to clear the ground, or it will self seed quite easily.
A cheaper and easier green manure is simply mustard but you will need to chop it down before it flowers and goes woody.
Clovers and things like lucerne fix nitrogen as well as covering the ground, but aren't as easy to germinate and take longer.
You don't say what you want to use the land for in the longer term? If you want the land permanently covered you could try growing a spreading ground cover plants like Houttuynia or Vinca.

you could grow a lot of higher plants close together . i have done this in my front garden and i hardley get any weeds anymore as they get know light.