It's all in the planning.Also, if you're going to change class to acquire certain skills (the most common being ninjutsu, the second being kirijutsu), you're much better off doing it early in the game. As it is, you've accumulated ten levels that you're now going to "throw away" with a short jaunt through the ninja class (as that gives you both skills you want) only to change (or revert) to something else.
As you've noticed, when you change class, your attributes drop to the racial/class minimum, whichever is higher. They don't go back to your original numbers. (Life's like that: you live within the parameters of the game; if you really don't like it, you'll just have to hexedit.)
You plan your class changes to make use of this fact. For examle, ninja is a very good transition class, as it has requirements for every attribute but one (personality). Thus, a ninja can change class to a samurai, ranger, valkyrie, etc., after only a few good levels.
Lord is one of the hard classes to transition to, as the only other class with a comparable personality requirement (bard) lacks any other overlap with the requirements for lord. You might compensate by making your lords hobbits. I usually deal with it by making them felpurrs. (The other solution is to skip the lord altogether, and take a valkyrie, instead.)
Plan your final classes. IMHO, to make optimal use of the prime, guaranteed equipment, you want a valkyrie, a fighter/lord/valkyrie, a samurai, and a bard. There is some good ranger equipment, too, but the class is only so-so for the end game, and the equipment isn't THAT good.
Also think of what you want to import to CODS (and ultimately to W8). Each character can only import one "prime" item, such as the Avenger, Fang, Elven Bow, Muramasa Blade, Cat O'Nine Tails, Bushido Blade, etc.
Finally, again imho, pure casters don't make good end-classes. If you don't want to mix spell schools, make your priest a valkyrie or lord and your mage a samurai or bard.