Sigh... I miss the old days.
Currently I play in a high/epic level campaign. We have at any given time as many as 8 PCs and a few NPCs. Between us all, we pretty much have every possible scenario licked. Sure, we face challenges from time to time, but tactics have become meaningless. No matter how much we try and plan things out, seriously bad guys can get the jump on us... they might have a magic item we had anticipated or some such. But with a group our size and our power level, we can usually cover for these situations. Besides, the worst that generally happens is someone dying. True Resurrection takes care of that. And as for humiliation - it is not really an option.
Rewind 10 years ago to my first AD&D campaign. A Bard, a Paladin and a Thief. None of us had great stats. Our DM was a stingy bastard... he kept a tight reign on our money, the availability of equipment and magic items. Hell, the first non-consumable magic item we came across was at 4rd level! Bloody magic sword that turned out to be evil so we threw it in a lake *mutter*mutter*
Despite our lack of resources, the DM threw some wild challenges at us from time to time. We were hungry for rewards so we sometime took fairly desperate risks. Desperation can make you do some pretty amazing stuff with your limited resources. My Bard's meagre collection of spells could be used to really great effect. Magic Missle can really only be used to attack a foe directly... Grease on the other hand has a thousand and one uses.
My advice - always keep your players hungry. Control their resources... magic in particular. When the party wizard tries to buy a scroll of Fireball, tell him Ye Olde Magic Shoppe is sold out, but has few Fly scrolls left. A favourite tactic of my old DM was to give Clerics/Druids different spells to what they prayed for. If some DMs think this is mean, consider this - deities *grant* spells. It is a not a divine right. A deity has every right to occasionally force upon a cleric/druid a more utilitarian spell.
Challenge them... ratchet up the CR on some of the encounters. Don't be affraid to even offer *overwhelming* challenges. There is no reason why encounters should be evenly matched. In real life we sometimes face challenges that are simply beyond our ability. Don't be affraid to present this to your players. Encounters don't always have to be fair. (This does not necessarily equate to killing the PCs... be prepared to give them options to escape). You can redefine the success criteria of an encounter as 'simply surviving'. You'd be amazed at what players can think of in the heat of impossible battles.
If you take hungry players and present them with overwhelming challenges you need to incentivate them with a glipse at potential rewards. Don't have them expecting bags of gold at the end of each adventure... but always have a sufficiently juicy carrot hanging from the end of your stick. However, don't always let them have the carrot or they'll become complacent and lazy. Keep 'em on their toes.
Hrrrmmm... this is starting to sound like training an animal.
