I Use DropBox 99% of the time. You can grab a free account through this link. If your mix session files are absolutely huge and your upload speed is slow, some artists and producers will fed-x me their initial mix session folder on a USB Flash drive. Using straight up FTP is going out of style but I make that available too.
The short video below will help if you’re unfamiliar with DropBox. You can use the browser-only DropBox and/or download/install the DropBox client-app that syncs with your computer. The video example below is Mac but there are lots of adjoining youtube videos you might find more useful like the Windows versions etc. The main thing to understand is that after you sign-up and upload your mix session files to your new DropBox account, you’ll send me an invite from your DropBox control panel with my songworx email address. You’re done!