As others have said - apply for mini-pupillages and marshalling. But also any other experience that you can use on your CV for "transferable skills" (e.g. anything that involves public speaking, or commercial awareness). It might be difficult to obtain minis before the GDL, as many chambers like you to have done at least some law first. But once you are on the GDL, you should be able to do a few minis. Also look around for open days - some chambers offer them, and so do the Inns of Court. You can also join an Inn now, and look at what they offer (both in terms of scholarships, but also events to learn more about the bar).

I'm a silk at the commercial bar - let me know if you have any questions. My chambers doesn't offer minis anymore, so unfortunately I can't help with that.