I did something similar to what Zolo is describing. I live in Wisconsin in a house built in 1951. It has 2x6 rafters, 2x6 joists in the attic. The attic is also a HUGE storage area, and I didn't want to lose it to cellulose, so here's what I did. I took about 60+ hours of labor to go between 23 rafters on each side of the house (44 spaces total):
1. Removed all the old cellulose from the floor. 30 55 gallon garbage bags. By hand. In hindsight, I'd buy an electric leaf blower/vacuum, 100' of 4" corrugated tubing, and suck it outside into a dumpster. I've done this before and it works SO well. probably would have saved me 10 hours. Minimum.
2. Air sealed the wall plate with great stuff/foam scraps (I pre-cut my foam - read on to see).
3. bought 1x4'x8' foil faced foam (about 15 bucks a sheet at Home Depot). Inches are at a premium when insulating the rafters, so spend the extra on the good stuff. It also cuts/installs way more easily) I cut it to fit between my rafters (about 14.5"), then used cutting scraps to make 1" spacers.
4. Glued/calked/GENTLY nailed these into place between the RAFTERS (to save the floor space/make this a conditioned space - not it only works if you vent EAVE to ROOF vent/ridge vent). I foil taped the seams in the foam boards and used great stuff (which is closed cell foam) to air seal it.
a. Make sure the foil is facing the roof. It'll direct heat up/out your ridge vent.
5. bought additional 2x6x12 foot lumber. Bought connection plates. Joined these to the rafters. This gave me about 9.5" of air space from the foam to the edge of the new board. I put some R-30 fiberglass in here. Total R-value: 36
6. took additional foam boards and sealed the top of the ridge (leaving about 6 inches of airspace).
7. Put foam board/great stuff on the gable ends. The attic is now air sealed.
8. Furred out the gable walls to also accept R-30 insulation.
9. Put another layer of foam board over everything to seal it from the inside. Foil facing INSIDE the house here. This will reflect heat or cold back into my living space. Aluminum tape EVERY joint. Now the attic is double air sealed, and R-42 total. I also have the entirety of the attic for storage. My wife likes to decorate for holidays so it makes it easy to store crap like that.
This has already cut my heating/cooling bills in half (I went from about R-19-23 to R-42/air sealed). It was running me 400/month or so to run either A/C in summer or heat in winter. I just did it in April so I can only tell you how it's affected the A/C but we've only had the air on for about 7 days so far this summer. The house temperature is much better regulated and not subject to huge midday or evening swings in temperature. And Wisconsin gets hot/humid here in the summer (Madison). This will pay for itself in about 6-9 months - it cost me about 1500 bucks, and took 60 hours, but I think I'll get a chunk of that back on my taxes and I'm already about 500-600 bucks toward the black with this investment.