You should check out this site. He is very helpful but you must read his requirements for activity on his forum very carefully. http://www.johnlsayers.com/phpBB2/index.php. Also this http://www.ethanwiner.com/acoustics.html
Bass trapping is the main place to start. I wont explain bass trapping here as you could spend months studying it but I can tell you that it's the number one way to clean up a room sound. You can apply bass trapping quite cheaply and there's lots of DIY methods on the net. A large mattress for example is an effective bass trap. Another good tip is unopened rolls of rockwool, yet another is a closed plastic drum filled with rockwool. ( the outer skin in these examples helps stop dampening high frequency reflections and this is usually preferably because if you were to dampen all the low with just heavy mattresses for example you would also remove all the high and mid and this is most likely not what you want as it's not representative of a typical listening environment ) There are several very different types of bass traps that you can use, some more ad-hoc, some more professional but like I said you will need to research all that yourself as it's a phenomenally big subject, and the basis of audio treatment from the little I know.
Flutter echoes and other strange sounds You really just need to have the confidence and patience to actually 'listen' to the room and work the problems through one by one. Make transient sounds in the room and listen and try to figure if there are any flutter echoes or strange sounds that may require spot treatment. You could treat these with something like some hi or mid dampening ( like a thin cushion or a professional version of the same thing ) .... or diffusion ( like a bookcase or some other irregular surface ... or a professional version of the same thing )
Hi and mid damping I think you would apply this last if you still find the room too live. In general studios go for the soft ceiling hard floor approach, but in my case I needed to go for hard ceiling soft floor ( carpet ) but if you're not recording it wont matter as much, but you'll probably need one or other to have some hi frequency damping and it's preferable to have a wooden floor with some cushion like material in spots on the ceiling as it's more controllable and you can get more into the mid range without killing all the highs as carpet is too thin so it tends to have the effect of killing all the highs and leaving all the mids which may not suit you.
Diffusion You can add diffusion if you want. I don't know a lot about it but I know lots of studios have a massive diffuser right at the back - it tends to make the room appear more open, sonically. I think if you're mixing position is near a wall ( behind you ) then you'd want to consider one or two. Home made ones can look good if you're good at woodwork.
Hiring Someone As an amateur myself who is happy with my own results I would say that there's no need to hire anyone, but as with everything a professional should bring a better result so if you have the budget go ahead. It is likely to get expensive though as I cant see professionals using ad-hoc methods ( even thought they work fine )
Software and Testing I bought a reference mic to do this and never bothered as I was happy. I have no intention of bothering myself with it at this stage but I can tell you that it depends on how sonically accurate you want your room to be. People go to great lengths to check and get things 'just so'. I would advise you to leave the measuring till last though ... at least until you have put in your bass traps. There's no need to measure before then ... most rooms require more bass traps than anyone's prepared to put in them. There are however 'nodes' which you can calculate and/or measure and you'd ideally have these for bass trap positioning. I'm sure there's lots of info on room testing on johnlsayers or ethanwiner's sites.
Speaker position very important. There's a lot to research here but in general you don't want them stuck against the wall. Leave at least a couple of feet behind and around them if you can. You also may want damping directly behind them, and possibly put them on some neoprene rubber or something to stop vibrations.
Noise I know you say you don't need soundproofing but your ears will be affected by noise, even subtle things like your pc's fans. You should do everything you can to keep noise down.
PS I notice there are suggestions of blankets and getting the room so that it doesn't colour the sound. I am not an expert but I do know that blankets are likely to cause you problems and also that you may not want a totally dead room. In theory it would be possible to suck all the sonic characteristics out of the mixing room but I don't think this is preferable. There is debate about this, but it's not usually possible to deaden a mixing room effectively anyway. Also note that if you use blankets, rugs, egg boxes etc you are likely to be masking the problematic bass frequencies making you think that the problem has gone. Ultimately you are looking for a clean room first ( no flutter and a balanced r60, not full of bass like most rooms are ) . Then you can decide how much more dead you want to make it. ( google optimum R60 for mixing your particular type of music )
