I'm super "type a" so take this with a grain of salt...
I look at my "schedule" and mentally walk through each day and calculate how much I'd tip for each encounter, each day and tally how much I need down to how many singles, fives, tens, etc. It's actually pretty easy to figure out. Then I get small envelopes, fill them with the amount planned, label them with the date, person, & amount, and place it in a ziplock bag. I also keep a larger envelope in there with a few extra bills just in case I miss someone or want to add to the tip I had already pre-decided. I keep that ziplock bag in our safe and then each day we just take out all the envelopes with that date. It really helps us to never feel stressed that we don't have the cash to tip AND we don't touch our "spending money" cash or vice versa.
Walk through your trip... cab drivers, porters, housekeeping (if they'll be a hotel stay), tour operators, room service...