I'm currently running a level 16 campaign that started at level 14 and will go to around 21 or 22.
The Pathfinder OGC is your friend http://www.d20pfsrd.com/.
In particular there is a monster builder in their bestiary section that allows you to advance most of the monsters from the Pathfinder Bestiary by adding the most common templates and straight hit dice increase.
There's also a great random treasure generator under the Extras section (at the bottom of the left most column).
There are a few things that have made this easier and more fun for me.
The first is remembering not to sweat precise details. Don't build full NPC stat blocks if you can avoid it. Give them the stats you need for their purpose in game. Base it off of something similar if you have it, if not wing it. Don't create a brand new monster if a repurposed monster can do what you want (use the stats for a white dragon to create a new ice demon for example).
The Second is ignore experience points. Keep a rough tally of the number of level appropriate encounters that it should take to advance to the next level and when they're there tell them to level up. If you stop tracking XP it greatly reduces your book keeping.
Third throw out the treasure by level guidelines and don't bother tracking it to closely. Start them at about half what the book tells you to give them. Otherwise your treasures will be minor bumps (from a +4 sword to a +5 sword, instead of from a +2 sword to a +5 sword). Realize that certain characters will get way more use out of certain items than other characters and so it's actual impact on gameplay is way less than it's gp value would suggest. For example, my current party is a paladin, a ranger, a sorceror, and a cleric. At this point they've all specialized in their chosen fighting styles. So if I throw in a +5 holy anarchic hammer I know only the cleric is going to be interested in using it and he almost exclusively casts spells in battle. So despite this being a +9 weapon by gp value it will almost never come into play. This can be a problem if you allow easy selling and purchasing of magic items since they might sell it, but I don't make it easy to do that so it hasn't been a problem for me and the treasures feel substantial. The trick is to keep their primary weapons relevant, but not overpowered.
Also don't be afraid to throw really powerful creatures against them or to kill them. At this level, they can handle almost anything. Also don't be afraid to throw underpowered encounters at them, you want them to feel powerful at this level. Just use what your story calls for. If they can't beat it they almost certainly have the power to escape, gather new resources, and try again.
I ran a campaign that went up to 14th level (this is actually a continuation of it with nearly all new characters) and I hated it because I tracked every detail meticulously. Learning to relax and just go with something approximate makes all the difference in the world.
Good luck!