I'm currently playing in a 3.5 campaign with an experienced group of players with a relatively inexperienced DM. We were having a bit of a cakewalk initially too. Just some things we pointed out to him:
- Challenge rating is very fickle. Dragons, Demons, Devils are generally overpowered for their challenge rating. A lot of other stuff is underpowered. To keep it excited use CR of party level +1 to +3 for most encounters.
- If your group exceeds the standard number of players (3-4), encounters get exceedingly easy.
- don't alow the adventurers to rest after each encounter. slow atrition of spells and resources can make the third encounter in a row epic.
- use terrain! make sure the terrain benefits the enemies, don't funnel them through a 10 foot corridor, etc.
- no problem with being stingy with (magical) loot
there was more, but can't really remember right now.