Wicht
Hero
That people keep repeating it's a DM issue that the casters keep outshining the non-casters is just sad... you're acknowledging that you actively need to build encounters around the fact that the non-casters only have limited ability to participate in the game and that casters have very powerful option, while the GM must force himself to create situations where not only these characters can participate BUT also show that they are required.
the scope of what non-casters and casters are capable of is so wildly varied that this GREATLY limits the scenarios I can put in front of my players since I can't get a good gauge on what they're capable of.
I don't actually think you need to build encounters "around the fact that the non-casters only have limited ability to participate in the game and that casters have very powerful option." In point of fact, this seems an unfun way to design encounters to me.
Rather, the best approach to designing encounters is to put in a variety of challenges in a way that seems pleasing and desirable to you the designer, and then let the players deal with it however they want.
You don't have to figure out ahead of time all the ways your players will solve the problems, because they will likely surprise you (at least they tend to surprise me).
However, if your design incorporates a variety of challenges, requiring more than one solution, then chances are much higher that the party will have to work together to solve the challenges. And while magic will be the solution to some of the problems, if you have done it right, there is little reasonable way it can be the solution to all the problems. Wizards just do not get that many spells per day.
As an addendum to this, when designing high-level challenges, it is far better, rather than trying to restrict choices, to design encounters which demand high-level abilities such as invisibility, fly, teleportation, or the like. Furthermore, as I have noted elsewhere, if your spellcasters are selfishly using all of their spells on themselves, they are being suboptimal. Create challenges which require, not a buffed up cleric, but a buffed up fighter. Challenges where even an invisible wizard is going to have difficulty, but an invisible rogue will get the job done.
Teams in which there is cooperation always outperform teams where there is glory-hogging in my experience.