For Dragons: buy the Draconomicon and use the meta breath feats. Dragons with quicken breath and maximize breath are fun. Also, dragons are smart. They usually know that adventurers will be protected from their energy type if people have seen the dragon. Plan accordingly. Dragons shouldn't be afraid to get in the middle of the pcs and mix it up, attacking all out with every weapon at their command. Remember to have the dragon use spells and magic items from it's hoard to protect itself. Shield and Mage Armor raise the Dragon's AC by eight points, very handy for not getting hit. Scintillating Scales, from the spell compendium, changes your natural armor bonus into a deflection bonus to help your touch AC. Cast Cat's Grace to improve your Dexterity score. If you have an energy weakness, use the best spell you have to shield against it (ex. fire shield, protection from energy, resist energy, or energy immunity from the spell compendium if you can cast it). You are a dragon, squish the lesser creatures like the almost god-like being you are.
Liches: are usually pretty bright. They should pick their battle spot very carefully, be surrounded by minions, have spells cast before the party arrives in the room with plenty of time to spare on their duration, and have nearly fool proof escape plans. Example: A lich's choice of battleground could be a maze filled his servants and deadly traps. Not only does the lich know the maze perfectly, he also has numerous phase doors scattered around so he can jump through a wall (or the floor or ceiling) and not be followed. Also in this scenario, use wall spells to separate the party into smaller groups.
Beholders: should also have plenty of cannon fodder handy. They should try to get into a position to use as many eye beams as possible. Think three dimensionally as they can always go up and over. Also remember they can turn upside down while airborne. Having a charmed low level arcane caster on hand makes for an invisible beholder hanging out near the ceiling.