There are lots of ways to structure RTWs. I use one ticket for travel over much of the year. Here's my example:

Oct 2004 - buy AONE4 in Cairo (ended last RTW there, so had no expense to get there)
- fly CAI-LHR-LAX
- spent several months at home

March 2005 - flew LAX-SAL RT
- home for several weeks

April 2005 - flew LAX-SJO RT
- home for 3 months

July 2005 - will fly LAX-HKG and then fly my Asian segments
- will insert a paid ticket CMB-HKG-LAX RT
- home for a few weeks

September 2005 - return to CMB on paid ticket from above, fly on to Oz and travel in NZ and Oz
- fly SYD-LHR
- insert cheap ticket (or mileage ticket) home

October 2005
- return on cheap ticket (or mileage ticket) to London
- fly London - Malta - London
- end RTW in Cairo
- pick up new RTW

Set up a spreadsheet and start working on various routings. You don't even have to decide initially if you are going to buy A or D class. Once you work out your itinerary, then you can see how many long haul flights you have that could be flown in A.

As for expense planning, determine how much it will cost you to get to the origination and then home again 10 or 11 months out. If you have miles, you might use an award ticket for this. Cost out the trips home that you want to insert. Add all of this to the RTW ticket cost.

It's a lot of work, and a lot of decisions - but oh, such a pleasure to fly.