I have been obsessed with this question in years past, and have tried a few different things.
I like those nylon and plastic rod tubes that have a pouch for the reel. A lot of budget friendly options out there come with this type of tube or you can pick one up for relatively cheap if your rod came with the other type of tube. I just put one of these in the water bottle/ side pouch deal on my pack. Also convenient that your reel isn't taking up space in your pack.
Check out the deal on Sage Ballistic Rod/Fly Reel Cases at REELFLYROD
www.reelflyrod.com
If that isn't packable enough for you, then you can get a rod with more than four pieces. I own a 5-wt, 9'0", 6-piece Hardy Demon Smuggler (best rod name ever), which is the best more-than-four-piece rod that I have ever used. They don't make them anymore, but you can still buy them brand new on ebay from a shop out of Australia. Hardy also made this rod up to 8-wt, so they have options if you are packing in to a salmon stream.
Additionally, I have a 3-wt, 8'0", 6-piece Douglas Upstream, which comes with a tiny tube. A great rod for little streams and dry flies. A lot of feel and finesse. It casts beautifully, but not very far. A great roll caster.