I’ll be brief.

1. Are you a female runner? Yes? Invest in a good sports bra.
2. Make sure it fits correctly. (There’s lots of good info on the intertubes re proper fit)
3. Replace said bra more regularly than, say, every 12 years or so.
4. Own a bra, any bra, that’s too snug? Save yourself $10 bucks and make your own extender using hooks from an old bra.
5. Watch this video for instructions:

   6. You’re welcome.