Kudos for asking yourself (and us) this question and your willingness to be a good dad. A lot will depend on the relationship with your ex-partner. I'm assuming a "reasonably amicable divorce" here.
- Step 1: Be informed about whatever is going on.
Make sure you are up to date with school events, sports club schedules etc. Shared custody is so frequent today that many schools have developed a "double information" policy for those cases, just ask to be informed. From experience, a shared online calendar can be a blessing - accessible for everyone, always up-to-date and avoids misunderstandings that can happen in verbal exchanges. In a few years, your children can even participate. Attend important and not-so-important events, even on your non-visiting days.
If the relationship with your ex is good enough, listen to what she can tell you about current fads and interests in the life of your kids. Talk to your children. Listen carefully and remember what they tell you. Ask follow-up questions at the next meeting.
- Step 2: Don't be a "sometimes" dad.
You are not the only parent who is absent from home for an extended time. Even in non-separated families, a parent may be away for shorter or longer time spans. Military parents, workers on oil rigs, people who travel for business... A short phone call, perhaps to say good night, Skype, even a postcard or (if you live close) or a note in the letter box are great means of staying present while absent. Be available, if the child wants to talk to you. Make sure they have photos of you.
- Step 3: Be a dad, not an uncle or babysitter.
If you get only limited time, you are probably inclined to "make the most of it" by spoiling them a bit. Which is fine per se. But I strongly suggest to also take over responsibilities - take over some tasks that the mother might usually do. Go shopping, but not just for toys, but mundane necessities like clothes, school supplies, whatever they need. Take them to the dentist's or hairdresser's. (Talk to your ex-wife about sharing those duties and shopping advice if necessary.)
Teach them basic life skills, just like you would have pre-divorce. You might have to learn a few things yourself, as you don't have the mom as a backup. You children will have to learn a lot of skills, from brushing their teeth to tying their shoes, from riding a bike to driving a car and painting a room. There is a lot of what you can give them that will stay with them for the rest of their life.
- Step 4: Be a rock. Be reliable.
Unless hell freezes over, try to be there for the scheduled visitations. If you must reschedule, communicate it clearly and as early as possible. You simply can't skip those weekends unless absolutely necessary. (This may be different if you and the childrens's mother have a more flexible system, but the base principle of the children taking first priority remains.)
Step in if necessary. If the mother gets sick, take over. Be willing to take sick days for the children, don't leave that burden to the mom alone. Even if grandparents or new partners are in the picture, be available and willing to make sacrifices in those mundane crisises.
One word of warning:
Even if you do all of the above and more, there might be a time when your children hate your guts and tell you so. It's probably not related to your divorce, even if they claim that. It's called puberty and happens in the best families. Remember the first mantra of child-rearing:
This too shall pass.
And the sage advice of my wise midwife:
Puberty is when the parents are getting weird.