Of the 3, Slope is the most straightforward (pun intended) but often missed. If your eavestrough is not sloping towards your downspouts, it will be holding water. This can lead to overflow issues, and quicker deterioration of the eavestrough itself.
Second is Sealant. Not only does the proper product matter, but also how it is applied. Silicone based sealants do not stand up well to UV damage, and will quickly wear down, causing leaks on corners and end caps. Proper care also has to be taken to ensure the caulking is evenly applied, with no air bubbles trapped, or areas where any water can get underneath.
Sealant also has to be applied at the right density. Too thin and it can crack in the winter, while too thick and you are creating a small dam inside the eavestrough.