Hmm - lessons I have learned two children down the line in a senior role in the City.
The hard ones:
Once you have a baby you are perceived as on the mummy train. I went from being nominated as high performer/train for senior exec role in financial institution to being on a redundancy list while on mat leave. I really regretted working up going into labour with that role, and I was totally unprepared for an at risk/redundancy process to slap me around my self deprived face.
So my top tip before going on mat leave is to assume you are not going back, have copies of any files/contacts etc/ resume up to date / and copies of appraisals at home. It will help with any arguments / interview processes.
You won't get the time back. I lived and breathed my job and never got that pre-labour massage and pedicure. I certainly didn't have time for it with No2 who arrived earlier and threw my lessons learned (take time off before due date) out the window anyway 
Plan for the unexpected but definitely communicate your intention to return and when that is.
Check your HR policies - they may insist that you use up accrued leave before your return. Your 6 months off may well turn into 7+ when you take that into account.
In my view give yourself 3-6 months mat leave. If you have a terrible sleeper [unless money really is no object and you can arrange a night nanny too] then until you sort that out, you will function poorly at work. With No 1 I was back and on a plane long haul at 6 months, with No 2 I was back part-time from home at 3 (No 2 was a solid little sleeper)
When you do go back to work, people will be surprised that you are back already even if it has been the longest 6 months of your life at home. Or they will have moved on and no-one will know who you are. Either way, everything will mostly still be the same and some of the stuff that was "urgent" before you left, will still be going on.
Live as close to work as you can manage. A shorter commute will mean you actually see your children Mon-Fri.
Enjoy the travel - it's a good nights sleep with room service. No point in beating yourself up about it. Leave time at the airport to pick up a gift for the suffering spouse at home who can't run out for a takeaway/pint of milk.
Find a good source of ready meals eg: Cook. and stash in the freezer for the nights when only one of you is there. By the time your kids are in bed and you are ready to eat, you'll just want something you can shove in the oven while you do bedtime stories.
Have low expectations of Skype/Facetime if your kids are under 4. They'll wave at you briefly and then beetle off to do something else.
Exercise is a means to an end - walk/cycle to work / run around the park with the pram while on mat leave. Whatever works.
Time for self beautification comes out of work time. Early/late/lunchtime apts are the only times you will get to do this. Go low maintenance on highlights etc.
KIT days are a useful way of reminding everyone that you are still around and exist. Use for strategy sessions like budget prep etc so you are not wasting your time. When joining these sessions though it is really tricky to take time out to go and express breastmilk when everyone is having a working lunch with some sandwiches. 
Bfing is a really useful motivator to get home on time in the evenings from 6-10 months. By 8 months you'll probably be weaning anyway but it helps in the first few months back to have that discipline.
Have them as close together as you can manage - it doesn't get logistically any easier until they are all walking and talking.
Be good to your nanny - you could probably never do her job as well as she does it. It's quite a long day with little adult conversation so make the time to chat and appreciate her efforts. Recognise that and be happy that your kids love her/him.
A nannyshare can be more hassle than it is worth.
Don't have kids with someone who is not prepared to pull his weight. Paying for extra staff is not the same - someone still has to manage them.
Grow a thick skin. Everyone will judge you for working full-time with small children.
When you have finished having children and they are heading to school age, consider moving to a new firm to kickstart your career again.