90% of planning is worrying about how to solve problems that might never occur.
Keep open minded, flexible, budget more than you need and be frugal with your spending.
Train in the first 2 weeks of your trip. Plan on the road.
Here are my planning rules:-
1. 1st hand recommendations trump guidebooks, maps, wikitravel etc...
2. time with locals, cultural experiences rank high.
3. if in doubt, head to the mountains.
4. worrying isn;t useful. making preparations, research, etc are.
5. if the shoot hit the fan, clean it up, get on your bike and continue on your way.
Many worries I had last year turned out to be ill-conceived. I worried about my health, money, language, the bike, border crossings, theft, etc. I was worried about Bolivia (high levels of poverty = potential for crime) and Colombia (cocaine and guerilla problems) but these turned out to be my favourite countries.
Some mistakes I made.
-Not understanding the complex currency exhange rate system in Venezuela. crossed the border and couldn't change USD. then there were two exhange rates: official and black market.
-getting lost, IMO this is all part of the fun
-not carrying enough water in Bolivia. this can get ugly. I should have found out in advance where you can buy water, carried puritabs, and carried a minimum of about 5 litres most of the time.
-not carrying enough food in Bolivia (some shops are bare)