We did two days at each house, then alternate weekends with a visit on the parent's off weekend. So she was with us every Monday Tuesday, with mom every Wed Thurs, then alternated Fri Sat Sun between mom and dad. That gives her 5 consecutive days with a parent sometimes, so in the middle of that 5 days the other parent would have a 4-ish hour visit. So on Mom's weekends she was with mom Wed Thurs Fri Sat Sun, but we would take her out for a few hours those Friday mornings. When it was our weekend she would be with us Fri Sat Sun Mon Tues and mom would take her out (usually to the park and dinner or something) for a few hours Sunday afternoon. (I hope that made some kind of sense... it looks logical on a calendar :)
It made the schedule pretty predictable for the adults (we could sign her up for a gymnastics/art/swim class on Monday or Tuesday or on Friday morning because she was always with us). We did have to count weekends when planning something, but I just wrote it on the calendar for the whole year.
It also made it easy to schedule a short trip or something for 5 days because it only required the other parent agreeing to give up their mid-weekend visit. That was pretty nice to not have to rearrange the whole schedule to go on a trip.
We did have to do some rearranging around holidays, but it was usually pretty easy to figure out. We allowed longer uninterrupted stretches for some holidays, which alternated each year, and we always found a way to split her birthday and alternate some other specific holidays, but in a lot of cases (like Valentine's Day, St. Patrick's Day, etc) we let things fall where they fell in the schedule and it always seemed to work out okay.
One thing we did to help her with the back and forth was to establish some really predictable transition routines. For example, the night before she went to mom's, I laid out the clothes she arrived from mom's in, which was a cue to her that the switch was happening the next morning. (It seemed to help to have something that didn't involve a discussion about it... telling her she would see mom the next day often led to her getting upset or getting clingy) Both parents let her know on the phone the night before what she would be doing during the visit or the first day of their parenting time so that she had something to look forward to (like we'd remind her she would have gymnastics, or mom would tell her that they were going to be baking muffins). And finally we each had a routine on pick-up mornings that we tried to keep consistent, like we always picked her up, ate breakfast at the bagel place, dropped me at work, then they went home to play.
One other little thing that seemed to help was that my husband and his ex agreed to let her walk from one parent to the other rather than holding her and "handing her off" to the other. The "hand off" seemed to make her cling to the parent she was leaving more, but letting her walk allowed her to run and hug the receiving parent, come back to the other parent, then make her move when she felt ready. The adults just chatted with her or shared something interesting about her time with them or whatever. It seems like a little thing, but it actually made a BIG difference to allow her to physically make the switch on her own.
We had a notebook and a bag of things that went back and forth (it was just the notebook, a stuffed animal or doll, a photo book with all her family in it, and occasionally something that needed to go to the other house) so she had some comfort items that were connected to both houses (the photo book was and still is big for her), and the parents didn't need to spend pick-up/drop-off communicating when she last had a bath, what she ate, or other info, they could just concentrate on helping her make a smooth transition.
Hope that was helpful, even if you have a different schedule in mind... we spent a lot of time tinkering trying to make it work for her the best way possible.