my husband Shekhar, has just got his results for skill assessment by ACS, well, we used an agent and i think he did a good job, a friend of mine did everything on his own. According to him and my consultant, just make sure you make an anexure, and put in all the papers that you hvae, anything and everything that is a proof of your academics, any course or education related certificate, pay slips to prove your employment, reference letters from as many people with respective visiting/business cards (co-employees, employer, seniors etc), previous employer reference letters and detailed description about work carried on by you by current as well as previous employers. Other than this, add all your papers like passport copies, marksheets, etc.
Make sure you do not put in any forged papers because they re-check from your employer and co-employees. Get them attested by a notary.
Keep duplicates of all copies you send them since you have to send the same copies again to the visa department.
Last but not the least, put all these papers in a file(the pocket file that we get, You can insert each document in each pocket with some kind of marking that can make things easier for them. Remember the easier you make it for them, the faster will your assessment be.
By going on your own you will save a lot, since consultants charge as much as the entire process fee is, but yes they do make things easier, you might just miss something or skip things without even realising.
And yeh, another thing, incase you are unable to give a reference letter or a detailed job description in your current company by your employer, you can give a declaration that it is not possible for you to give the same since you fear loosing your job, we gave it since shekhas had to face big issues at work because he asked him to sign a 3 month notice period bond incase he wants the letter.
Hope this helps.