Unless you're taller than average yourself - eventually you are going to find yourself matched up with someone you literally have to look up to.
Boxing a taller opponent has some unique challenges.
The tall opponent will usually have a significant reach advantage, longer legs, and a target area that is not where you are used to hitting.
It is absolutely necessary that when you do decide to go to the taller opponent that you do it with authority and decisiveness. Do not hesitantly walk towards him -- explode into him, push him back, corner him and punish him. Do not go in on a straight line, but rather duck, weave and whatever else you can do to give him the fastest moving target he has ever seen.
To get a good visual of what you need to do, take a look at this fight between Mike Tyson and Larry Holmes.
Holmes has a decent height advantage over Tyson and you will notice how Holmes attempts to pick Tyson off with jabs from the outside. He lets Tyson come to him (which works with Tyson's style anyways).
Tyson explodes towards Holmes at every opportunity and attempts to stay there to unleash a few combinations.
You will see how Holmes defends against this by tying up Tyson - even going as far as pinning/holding Tyson's arms under his own to nullify Tyson's ability to punch once he defeats Holmes' outer guard.
This should give you a good idea of the kind of explosion required to get inside and the work it is going to take to stay there - especially against anyone who knows what to do with you when you do get there.