I would also advise the above, including not having babies as from experience (I had three oops) and it was extremely hard!
If you are a parent you need to work out what time you can allocate to your studies and be efficient with your time. I found I had just evenings and one childcare afternoon each week so for part time study I could manage 15-25 hours a week. To accomplish this i needed to be really organised with lists of things to do and my own deadlines.
Find out early on what paperwork needs to be done for when, ie project approval, ethics approval, confirmation of candidature etc and note the meeting deadline dates for this paperwork. Aim to work towards these to keep on track.
A referencing system is essential as you will have lots of articles. Try to write notes on each article you read and its relevance to your work. File the paper or electronic copies in a logical order of subject and sub categories etc. Also as previously mentioned enter these onto a referencing software (endnote or refworks - provided free with uni access).
Keep log books of all of your data and meeting minutes, also you own thoughts about the direction of your project. These will show your PhD journey. These can be started now in the planning stage too. NB I used a couple of hardback A4 notebooks for mine.
Have a good PC/laptop with an external hard drive to back up onto. Back up your work regularly.
A PhD is a rollercoaster of mixed emotions and determination to get you through. It is common in the first year or so to feel overwhelmed and not really know what direction you are taking. Your research will evolve over this time bringing it to a very specific area and hypothesis.
A couple of books for interest I found useful were:
Estelle Phillips and Derek Pugh (2002) How to get a PhD
Stephen Marshall and Nick Green (2004) Your PhD Companion
Hope this helps!