You can re-specialize your character, or any character in your party.
While the core game does not provide a way to easily do it by default, you can download this modification from the BioWare Social Network’s projects area. In reading up on this topic, I found that is not recommended to use the console to do it as some have. I’ve used this mod and it worked flawlessly, so I recommend the mod install route, as it works just like installing DLC does. It places a Raven at your camp (or by the load in area for the Flemeth’s hut area) that you can talk to for a re-spec. You select the first dialog option to get a potion of re-specialization that you can use to relearn all those skills and talents, so if you messed up and explored a specialization or talent that you regret, you can correct it. If you are like me and think Morrigan’s shape-shifting skills are not as useful as the Spirit Healer skills are, you can correct that too. Just make sure you’ve unlocked a specialization before you unlearn it, or you’ll have Allister unable to learn how to be a Templar like I did. Luckily the book for that one is at the camp’s vendor!
Tips: You can re-spec Ohgren from 2-handers to shields for tanking, since you have to take him to certain fights in the Dwarf campaign. That way you can sit Allister on the bench, especially useful if you want to keep 2 mages in your group. Also, there is another download in the projects area that will put a chest in your camp that you can use to store items.

You only need basic Health and Lyrium potions, and you can have them in unlimited supply.
With basic herbalism you can make these crucial potions in nearly unlimited supply. Go to the Elf (Brecilian Forest) and Mage (Circle Tower) campaign areas and talk to the quartermaster/vendor there and buy Lyrium dust and Elfroot in unlimited supply. In the tavern in Denerim you can buy unlimited flasks. Buy all the stacks you can afford and crank out as many potions you need to ease the burden of healing, crowd controlling and damage on the Mage(s) in your party. Your Rogues and Warriors will be able to use the health potions quite regularly as well. Rinse and repeat as you run low on potions. There are areas where you can buy unlimited reagents for the larger potions if you wish to go that route, but that is a more expensive thing to do and you can get by without them.

Tip: Need room for all those potion materials? Check the full item list of any vendor you interact with, some of them sell backpacks that will add to the total amount of items you can hold at once. The Dwarf vendor in your camp has one.

Two Mages are better than one, so pick up Wynne as soon as you can.
Wynne is a playable party member you pick up during the Circle of Mages campaign. I recommend going to gather that part of your army first since she is there, and so is your unlimited supply of Lyrium dust for mana potions. Wynne is already specialized into Spirit Healing, but you can change that (see above) later if you wish. With Wynne and Morrigan re-specialized in Spirit Healing for group support you will struggle less to get through the tough encounters. You won’t regret having not one – but two – resurrection spells to cast in some of your fights. If you don’t mind tripping over traps and leaving many doors and chests unlocked because you don’t have a Rogue, you can play a Mage yourself and have 3 in the party!
Tip: Get one Mage into cold and one Mage into telekinetic spells and have your mages sustain force and cold weapon buffs for your Rogues and Warriors.

Crowd control is key, and AOE damage is icing on the cake.
Mages are pretty much the way to go here (see two mages are better than one above), but which skills you choose for Mages is even more critical. Force field, Cone of cold, Crushing prison and other spells like Blizzard and Fireball for AOE damage also have crowd control components. You can light the slippery AOE grease spell on fire with any fire spell for the double whammy. Blood magic specialization has a great crowd control spell, with quick re-use for creatures that have blood – like Darkspawn. Its not only Mages that are useful here, as your tank’s shield bash or two handed pummel will knock enemies down for s short period. Shield bash can also interrupt that Ogre from pounding your healer into the Dust too.

Tips: When faced with a group of nasties coming at you, one or two of them are usually tougher than the rest. Look for baddies with a yellow or orange colored name, and use force field or crushing prison to take these guys out of the fight while you kill the rest. Better yet, when a Mage (or anyone in your party) gets aggro and is about to die, have another mage put a force field on them since you can put it on party members to make them invulnerable as well. When the mob leaves, have the other mage dispel the force field to put them back in action!

Learn to use your tactics, and the pause and escape keys are your friends.
I started using tactics way too late in the game, but these basically allow you to program the behaviors of the party members you aren’t controlling at that moment. You can set Mages with healing spells to heal anyone under 75% health, and you can have Mages drink potions when under 75% mana – automatically. Have your characters apply the buffs they have automatically with tactics too. There is much more you can do to program behavior so that these common things you’ll constantly do will happen without you having to do them. The fights can get large and hectic with many things happening at once. Not only will tactics help, but so does pausing. Use the pause button (space bar by default) to plan out every move you need to make to get through a tough fight. Also, use the escape key to skip dialog. While the story is great, there are some parts where you just don’t want to hear it. When you go to the same vendor 20 times in a game, you probably don’t want to hear his diatribe just to see what he’s got to sell.

Tip: You can set the game to automatically pause the game when combat starts. This will help you visually locate and assess the bad guys. Preparing for that Mage’s fireball by running other people out of the way or crowd controlling them to prevent it are much easier things to do when you pause first. Having the game automatically do it for you helps you get a jump on the action.