Upgrading your Town Hall too early makes you a massive handicap in Clan Wars (it'll move you up the war ranking and you may not be able to compete against your opposite number), and may hinder your ability to loot well when raiding too (since there is a loot penalty when attacking bases with lower Town Halls). Consequently we encourage members to speak with a Co-Leader before setting off a Town Hall upgrade.
Some general advice is provided below on what to be thinking about before and after upgrading your Town Hall.
WHEN: The main thing that identifies you're ready to move on from Town Hall 7 to Town Hall 8 is that you should have L2 Dragons, be three starring any other Town Hall 7 base in war without fail and be regularly three starring low Town Hall 8 bases. Also be able to loot efficiently against other Town Hall 7 bases (e.g. with barch). I'd recommend also maxing out as much as you can troop and defence-wise but it isn't a disaster if you haven't.
WHILE YOU UPGRADE: Try to fill your storages as full as you can. This will help you upgrade key things early.
AFTER THE UPGRADE: Focus on army related upgrades first. Upgrade your Laboratory and Clan Castle as soon as you can. Then focus on upgrading your dragons, balloons and all spells. Also upgrade your new Air Defence early, to help fend off dragon attacks in war. After that focus on upgrades that help you loot (e.g. barbs, archers, giants, goblins, whatever your preference). Slowly start upgrading your defences whilst upgrading things like Hog Riders, Golems and Pekkas to make other war attacks available to you.
WHEN: Turning TH9 is a difficult transition. War attacks become more complex, and whilst protecting your resources becomes easier, looting becomes harder. To make the transition as easy as possible, it is best to max out everything you can at TH8. Walls are not critical, but it's easier to do them at TH8 than TH9. You can get away with leaving BK at L5, and not upgrading dark troops (except hogs which are a must, and ideally Golems) if you're struggling to gather DE fast enough. But things will be easier for you at TH9 if you get those things done first.
Additionally, make sure you have had plenty of practice with a TH9 war attack. GoWiPe is useful starting point at TH9 that'll help you secure safe two stars. Hogs are very powerful at TH8 and well worth learning as they'll transfer nicely to TH9 (particularly GoHo) giving you opportunity to three star some lower TH9s straight away.
WHILE YOU UPGRADE: Try to fill your storages as full as you can. This will help you upgrade key things early. Make sure you have at least 40k Dark Elixir so that you can get your AQ as soon as you are upgraded.
AFTER THE UPGRADE: Focus on army related upgrades first. Upgrade the clan castle, spell factory and get the AQ straight away, with your saved resources. Get the lab upgrading and start work on the army camps. Also get the new AD and upgrade that a bit, to help make sure low TH9s still using dragons can't three star you. X-Bows have a big contribution to your war ranking; don't get them until you're an established TH9 attack-wise. Initially your job in war will likely be two starring TH9s with GoWiPe, and cleaning up TH8s that need it. To try practice three star techniques like GoHo and GoLaLoon once you have the troops, in situations where it won't be costly for the war result if you fail. They take time to master, don't expect immediate results.
WHEN: I would max out everything at TH9 except walls and heroes. Big heroes make a big difference though, I'd get them both to L20 before upgrading, certainly at least L15. Walls do take a long time, but try to at least reach all L8 walls with a solid core of L9 walls. L10 walls do cost a lot but they have a big gain in HP, so can be very useful for getting troops stuck in your core. Try to get some practice in against TH10s before upgrading (low TH10s with two multi infernos can be taken for two stars with a well-planned GoWiPe, ones with single infernos can be taken with Giant/Wiz/Valk/Witch type troops).
WHILE YOU UPGRADE: As usual fill up your storages as much as possible whilst your TH is upgrading.
AFTER THE UPGRADE: As usual, focus on troop related upgrades first. Upgrade your clan castle with your saved gold, and lab, army camps and spell factory as soon as possible. Start upgrading your freeze spells as soon as you can. If you are struggling to two star low TH10s hold off on your new X-Bow and Infernos else you'll be making matchups harder for yourself and your clan. Once you can two star TH10s though, I'd get them and start upgrading the infernos as they make it much harder for the opposition to two star you. Sometimes in war you'll be dropping down to TH9s, make sure you don't do something a TH9 could do. In our wars we rarely three star all TH9s, so dropping for a TH9 three star is normally good value. Only drop for two stars if the clan doesn't have enough TH9 attacks left to cover them.