Well... rule number one while soloing is that you solo
This means that you do ALL things (and that means *ALL* things) by yourself. This involves not allowing party members in you party, not even when they're needed (i.e. when you need to have them in the party to get a quest, you take them, go get the quest, and ditch them as soon as possible). As Dundee said; Valygar is the only exception in this principle (AFAIK the other quests [such as de'Arnise Hold] can be entered without taking the NPC with you).
In short: soloing is finishing the game without the help of other party members. No XP must be lost because you have party members.. The only disadvantage, is that you don't get the party member-related quests (such as the quest for Jan Jansen).