I spent over 12 years making jams, marmalades, chutneys, conserves, etc. every single day, and here's the process we used.
1) Clean jars every time. They'd come direct from the factory, but we'd sterilise them in boiling water for 15 minutes before using. You can happily re-use a jamjar over and over again, provided you thoroughly clean it and then sterilise.
2) Clean, new lids/caps every time. Lids have a latex or plastic seal inside which provides a waterproof/airproof seal when the lids are twisted on. We'd briefly wash the lids in hot water (not boiling -it might damage the seal) before use. Once that lid has been used , then the seal is compromised, so we never used it again.
3) Once the jam/marmalade/chutney/conserve is cooked, jar/bottle it immediately, piping hot. Clean the rim of the jar (this is where mould frequently forms if residues of the contents are left on the outside) and then twist on the lid/cap. Modern caps (not the Keeler or Mason jars) have a little "button"in the middle which will "pop" when a vacuum is created inside the jar. That way you know the seal is done. Turning the jar upside down has the same effect but, to be honest, is not necessary if you've already filled the jars with hot jam and sealed immediately. I only do that for home preserving; ie. pickled beetroot, Middle Eastern pickles, etc., where I re-use mayonnaise jars. If you turn the jar upside down, then the lid is saturated with what ever is in the jar and this, eventually, once the jar has been opened, might accelerate rusting on the lid, and therefore affect the shelf life.
4)Leave the jars for 24 hours (so they can cool down) or overnight, and then store.
Once a jar of jam/marmalade/pickle etc. has been opened, I keep it in the cupboard, not in the fridge. There's more than enough sugar/vinegar/spices to conserve the content, and I can assure you that cold marmalade tastes nowhere near as good as room temperature.
Lids and sugar concentration are important. I never use brand new jars - I reuse jars from three types of jams and fruit we regularly get that have really good lid seals. 
