This recommendation was kindly submitted by Mango and assumes a working knowledge of 'swapping'.
Figure out what class you want your character to be at level 30. Now consider what its weaknesses will be - will it have too few hp, or too few mana.
Now choose another class which will rectify that weakness. For example a warrior has too few mana, so I'm choosing a starting class of mage.
Play your mage to level 7, training as normal. Also practise any skills you will need later on - for example armour and maybe one offensive spell. When you reach level 7, swap to your destination class. You will continue to get the hp and mana increases of your first class right up until level 14. After that you will get your destination class increases.
I have a mage character who started out as a warrior. She had 402 hp and 299 mana at level 36 and plays very well. If you want to get slightly smaller hp/mana shifts you can swap to your destination class at lower levels.
I wouldn't recommend ever swapping more than once.