Pack light, of course
Three changes of underwear/shirt seems to be enough. Wash what you've worn on the road in the shower with you every day. You'll also need a warm fleece that fits under your riding jacket, and waterproofs.There are loads of packing lists on the web. We all bring too much with us, and then gradually dumpit along the way. It's part of the fun.

Two pairs of trousers
Rohan in London make a fantastic pair of trousers in super-strong super-light material. My Cargo pants have lasted two years being worn most days. After a wash they still look decent. They contain kevlar, and cost a packet. But cheap in the long run. They've withstood everything except an attack by a wild dog in Thailand.

On the feet
A pair of riding boots that you can also walk in saves a lot of space. My CAT boots took me around the Annapurna circuit and all the way to Australia in a lot of comfort.

The protection level is very small though; and overlanders I know who have had serious accidents while I have been travelling have all hurt their lower legs badly. There is certainly a strong argument for proper protective boots, that I really should heed.