In one sense packing is very easy.  Relatively speaking, you have so little space that such limitations, by necessity, limit what you can bring.  Pounds matter, and packing light is a principle to which any seasoned traveler always tries to adhere (though not always successfully).  

Yet on the other hand, packing light requires tough choices, and this is where it gets difficult.  Do I really need that 3^rd shirt, or will two do?  Is that telephoto lens a necessity, or merely and expendable luxury?  Will I react violently if, during an Indonesian monsoon, I look in my pack only to realize that my Gortex parka did not quite make the cut?  You get the picture.  

Take a look about what made it into our packs at 2 Go Pack List, including 2 Go Toiletries and  2 Go First-Aid Kit.  

The process to us involved, like everything else, a  lot of reflection and reading.  Consider, first and foremost your destination(s) and the weather conditions you will likely encounter.  Not much use for that fleece parka if you're spending your time on the beaches of Southern Thailand, but if you're trekking in the Nepal Himalaya, you better bring the fleece and a whole lot more!  Our journey complicates things, because we will see all climatic extremes.   

As we debated various item, certain  were clearly indispensable, and had to be included regardless of their weight or bulk.  Early on, we made the decision that our computer and related equipment fell into this category.  Similarly, anti-malarial and other prescription medication fell into this category.  So did at least 4 pair of underwear.  

Read guidebooks, and see what the authors suggest.  What you bring is largely a matter of personal preference, so think long and hard about each item, and whether you really, really, need it.  Not that you might need it, but that you will need it.  When packing think, as did we, of these maxims:  

If it doesn't fit, you must acquit.  –Johnnie Cochran  

If in doubt, you must leave it out.  –2 Go Global  

Actually, the second maxim was, to us, the more useful.  Look long and hard at each item and, if in doubt, "you must leave it out."  Sadly, we have no doubt violated this principle ourselves more than we should have.  Oh well.