Having divorced parents (I'm now 29), some things to keep in mind are..
1) Don't ever badmouth each other (together or separate) in front of the kids regardless of how insignificant or discreet you think it is - even if at first they don't seem to mind it, it will eventually harden them anytime the topic of 'dad' comes up. My mom was great at that... your dad could never seem to finish anything... he was never around... his work was more important... he played favorites to his kids... he always made me the bad guy... so on and so forth
2) Try not to treat the relationship as a mistake. He was a big part of their lives (even helped create one of them) and they don't need to think badly of that time. In some ways it translates to kids that those years in their childhood were mistakes. The more bad things either of you say the more it will affect their happier memories - and they deserve to have those :o)
3) Point out the good in divorce. You are showing them that they should never allow themselves to be in a situation that makes them unhappy and that there is always something they can do to change it. You can point out that you would hope they would do the same because everyone deserves to be happy as long as they have done everything they can to try to make the situation work.
4) They don't ever need to know the details of the divorce or what caused it. I know you may be hurt and need to vent (I was 14 when things got really bad at home and my mom talked a lot to me, which I was fine with at the time - even thought I could help - but over the years it got harder and harder and now I hate it any time she even mentions my dad's name to ask how he is) but don't... chances are they know a lot of what was wrong and are best left not knowing the rest - especially if dad wants to remain a part of their lives.
Also, if you are able, work with your soon to be ex to be civil to each other and do things for your kids.
Some things you may try on the home front would be taking time at dinner to talk about their day and stay in touch with what is going on in their lives (my husband's family always spent 30-40 minutes after eating just sitting and talking - each kid got a turn to tell about their day and what they were doing in classes and how they felt about things - and they are one of the most well rounded and in touch families I know.) Another thing you can do is designate one night and pizza and fun night where you can play games, watch movies, do a craft, anything as a girls night together. This should help your girls hopefully stay secure within their new family setting and know that you are paying attention to them and care about their lives. Stay involved! :o)
Hopefully this helps a little...
Best of luck!