I'm sure there have been other similar posts on this subject. The latest one I saw was in 2006 so maybe there's new feedback out there that will be worth sharing.
I bought my Blazer new in 2005. I never would have thought of bying one till then because they used to be priced way out of my range. But they were giving then away because the model was being discontinued. So having saved a few bucks I opted for the dealer's rustproofing package. The stuff they put on (some rubberized compound) fell off after the first year.
I decided to protect my investment and went to Krown rustproofing for the two following years. Three weeks after my second annual rustproofing I had to get under the vehicle to do a regular transfer case & differntials fluid change and noticed that there was an awful lot of areas that had not received the Krown treatment and were showing early signs of surface rust. I'm talking frame and suspension and steering components. I will not argue that Krown is a good product if it is properly applied but It seems my local Krown dealer was cutting corners. I decided that I would do my own rustproofing. The Krown broshure I had explained that the secret of their product's creeping and penetrating properties was wool wax oil. So, I found a product called Fluid-Film which is wool wax oil based. For the price of one Krown treatment I can buy nine cans of Fluid-Film. I only need about three to treat my Blazer. Plus you can buy it almost anywhere around here. (I would like to state that I am not trying to glorify this product. If I find something just as effective and cheaper, I will switch.)
Now I don't just limit myself to one rust proofing per year. Every time I do anything on the vehicle I have a can of the cheaper stuff on hand and do spot treatments where needed. It dosen't just cost less, it's more effective rust proofing. Since I like to keep my ride clean, I usually wash it about twice a week. This tends to wash off the rust proofing. Regular spot treatments are an answer to this as well.
My thinking is keep your Blazer in good condition because they aren't making any more of them.

 

I do a similar thing, I always have a can of undercoat and have touched up the underside of my truck. I have also taped off a 1.5 inch section on the bottom of the truck (the part that runs under the doors) and given it a few coats of undercoating. It works great and keeps it going. As for not making them anymore when its time to get a new vehicle I'm going to try to import one of the Brazilian Blazers haha.

 

I put mine on jack stands, took a wire wheel on a cord drill and ran it for about 3 hours to get all the rust off of everything and then i used POR-15 paint on undercoating. It works awesome and it gives it a good look. A bit expensive but apply it right and one can will cover a decent area. After i POR-15 it i usually take hi-heat heavvy duty spray paint and paint the fram and some areas that are visible to make it look nice and shiny.

 

i second 9blazer6, i did my entire frame and underbody with por-15, it is an expensive and messy job, but its only one time, and it doesn't come off ( including your, skin i found a chunk on my hair months after i did it). I even put it in the wheel wells and it has yet to get a scratch/ding. Its also a good idea to use one of their topcoats because it is just as strong, and protects the por-15 (which it doesnt really need :P)

 

I am also planning the same. Gunna snag a few wire wheels from harbor freight. Last time I did this to a 69 camaro I found bits of wire stuck into my shirt. Wear eye protection if you like seeing. I coat it with some rust preventative etching primer and then undercoat. This is going to be my fall project before the snow and salt comes

 

Used motor oil in a paint sprayer. Makes a mess, but tried & true. And free ...