There's another answer here claiming that an off-axis aperture mask will improve things. That is a fairly popular meme, but it's entirely incorrect, and makes misleading claims about the effects of the central obstruction. All that does is it hides existing problems with the instrument.
If the off-axis hole appears to improve things, something else is very wrong and you need to fix that first; it's not a solution, it's a symptom pointing at serious issues. More importantly, it reduces precious aperture, therefore capping the resolving power of the instrument in a situation (observing Mars) when you need all the resolving power you can get. Please don't do that. Ignore the off-axis mask altogether.
Now, what to do to get better results with Mars?
Make sure your telescope is in perfect collimation. There is no one technique to rule them all. There are many ways to collimate a telescope. Whichever method you choose, make sure you actually use it before observing Mars. Small deviations from perfect collimation will affect results on this difficult target.
Your instrument's manual, or the manufacturer's website should provide some info. Here are some links to get you started:
Some info about the importance of collimation:
Regarding laser collimators, which are very popular: A laser collimator is very quick and can be very good, but provided the laser itself is perfectly centered. Plug the laser into the focuser, watch the laser spot on the primary mirror, and slowly turn the laser like a knob - if the spot moves in a circle as you turn the laser, then it's not centered. Some lasers can be adjusted until they are centered (2 or 3 adjustment screws on the barrel), others can't. A laser that's not centered will not provide good collimation for your telescope.
There are many other collimation techniques, keep learning.
You don't need to be in perfect collimation every single time - a lot of faint fuzzies (galaxies, etc) don't require any careful collimation. But for Mars and the Moon you better do it right. Don't obsess over it, but try and do a good job. Personally, I at least check collimation every time no matter what, but with practice now it only takes me 5 minutes to fix any deviation, so it's not a chore.
If your telescope is not at the same temperature as the air, convection vortices will appear inside which will affect performance. Take the instrument outside at least 1 hour before the observation and let it vent out all the heat.
Some reading material:
This is basically atmospheric turbulence. When it's bad, the image is distorted and fuzzy. When it's good, the image is clear and you can see small details. Basically, Mars is impossible to watch if seeing is not good or very good. Sometimes, spells of bad seeing can last for days or weeks. There's not much you can do except put the scope in the car and drive someplace else where seeing is good at that time.
Seeing is basically weather-related, so it can be predicted to some extent. Go on the Clear Dark Sky website, search for a location close to your home, and pull up the chart:
Look at the row called Seeing. If it's dark blue for the time when you plan your observation, chances are good that seeing will not get in the way. If it's light blue or white, the forecast is not good.
Mars is only worth trying around oppositions, when it's closest to Earth. This happens roughly every two years.
The previous opposition was in July 2018 when Mars had an apparent size of 24 arcsec.
Currently (November 2018) the apparent size of Mars is 11 arcsec, less than half the size at opposition.
The next opposition is in October 2020, when it will be about 22 arcsec.
A few weeks before and after opposition Mars is worth trying.
Make sure the scope is perfectly focused. Point it at a fixed star (Polaris is quite convenient since it does not move), and adjust the focuser until the image of the star is as small as it can be.
It's pointless to try fine focusing if the scope is not collimated (see above). Collimate first, then you'll be able to achieve a sharper focus.
Also, if there are seeing issues, or thermal issues, a sharp focus will be hard to achieve.
Mars is a difficult object to observe. Good quality optics are important. Aperture is important (more is better - please disregard the memes about limiting the aperture). Experience is important - you'll get better in time.