Mars, however, needs a bit more magnification if you're going to see it at its best. Even when it is at its brightest and closest, as it is right now (March 2012) and will be every 26 months.
Mars, with a range of small, delicate details that require magnification to see.
The largest details on Mars--the polar caps and Syrtis Major--can be seen at magnifications as low as 35-50x depending on your viewing conditions. But Mars will show far more if you can get to 300x or something close to it.
But doing this takes more than a high power eyepiece.
First, we have to have a sturdy mount, which I talk about in many posts, including the linked post. Next, we need to have some way of tracking the sky. To work at high powers, your scope needs to have some sort of automatic drive mechanism, whether it's a computerized clock drive or a kitchen timer with a friction wheel pushing your telescope along. It should work smoothly, and for periods of time at least 15-20 minutes long without needing manual interference.
Then you're ready for a higher power eyepiece or a Barlow (which multiplies the magnification of your current eyepieces.)
But what if you have a Dobsonian or some other manually moved telescope?
In that case, you'll be limited to medium level magnifications, and you won't be able to pick out as much detail. That's because you won't be able to just sit and relax at the eyepiece for as long. I regularly observe through my all-manual Dobsonian at magnifications up to about 250x. It's not as pleasant as deep-sky views at 56x, but it can be done.
You could modify your scope, but to be honest observing detail on Mars is a pretty specialized pursuit compared to working your way through a variety of deep sky objects at low to medium powers. It's not worth taking away from a scope that does that well just to look at Mars.
Instead, consider getting another scope. A smaller one will do, Mars is plenty bright. That keeps telescopes from taking over the household with their size. Plus, a smaller scope can be driven by a smaller drive mechanism. Just make sure it's got a high quality mount that keeps it steady.
Also, the sort of scopes that have good optical qualities for looking at the deep sky are not as good for objects like Mars. They usually have "fast" focal ratios. That is, their f/-number is a small number--six or lower. Scopes with higher f/-numbers, f/7 or higher, will generally give better contrast and detail in views at high powers than a light bucket with an f/4.5 focal ratio.
On top of that, an unobstructed optical train, that is, one without a secondary mirror in the light path, gives the instrument an advantage. An unobstructed 3" to 4" scope, on something like Mars at high power, will perform as well as a 6" or larger telescope with a secondary mirror in the way. Like a Newtonian or Schmidt-Cassegrain.
This is why we can't expect to buy one telescope to do everything. Different scopes do different things well. If we try to make a scope do everything, it becomes compromised for everything. It's better to have a scope that does a lot of things very well, though it's not optimal in other areas. It's best to accept that, enjoying what it does well.
In fact, the best scope isn't necessarily the one that gives the best views. The best scope is one that fits your lifestyle and needs well enough that you take it out and use it a lot.