manually, one invoice for each client end of month with all projects listed, even long terms ones are paid each month so no big shocks down the track
they pay end of next month, i discussed this with them when we settled down to a working relationship, i don't put up with nonsense and am up front with them about it, once you get that out the way a working relationship is easier, more productive and it makes my working life pleasant as it can be
Online, with auto reminders built in that get sent every 7 days. As it even says "this is an auto reminder" they get the annoying reminders but don't take it out on you personally. It does seem to make a difference v my old manual ways (plus much easier for account keeping!).
I use billbooks.com after using Zoho for years. Billbooks you pay per invoice which worked out much cheaper than a monthly subscription for me, around half.
This is an issue. I give at least 14/15 days, I think 7 days is expecting a bit much, and if it's larger clients they often have a monthly billing cycle. Find out what the cycle is and make sure you get your invoice in before the end of a cycle or you'll be waiting 2 months.