I've always been surprised by how much some experienced (non-camping) backpackers carry around with them. 75+ Liter bags seem to be quite common. So when I realized that all my stuff - minus what I'll be wearing - only fills about 3/4 of my 50 Liter pack and weighs about 12lbs, I was quite pleased that it was so small and light... but also a little worried that I might not be packing enough.
This will be my first "long" backpacking trip (3 1/2 months) but i plan on taking pretty much the same stuff I used on much shorter trips (figuring that I can buy replacements over there).
My 50 Liter backpack is to get from hostel to hostel, but it's too bulky for sightseeing so I also packed a canvas day bag (to carry my camera, notebook, bottle of water, sweater, etc) that can be slung over one shoulder.
The following list only includes items which will take up a significant amount of space...
1 pair of khakis
2 pairs of shorts (one doubles as swin trunks)
1 pair of walking shoes
1 pair of sandals/flip flops (like they mated and gave birth to a hybrid... so they're good for walking AND showering)
1 long sleeve shirt
1 short sleeve shirt
1 dress shirt + tie (required for a job I got over there)
1 thin sweater
1 small spring jacket
4 pairs of ankle socks
2 pairs of underwear
1 sleep sheet
1 travel towel
1 hygiene kit
1 small notebook/journal
+ various odds and ends like hometown souvenirs to give away as gifts, deck of cards, mini alarm clock, paperback, etc.
This seems like enough to me. I'm even thinking of leaving behind either the long sleeved shirt or sweater because I'll only be there in late spring to the end of summer so it won't be all that cold.