To avoid an unfounded charge of illiteracy, you might feel the need to avoid splitting your infinitives, but don't feel obliged to.  If there is no way to avoid the split without introducing ambiguity, go ahead and split without regret, and refer your uninformed critics to Fowler.  He called slavish obedience to such unfounded rules a fetish.

A Daily Telegraph report (27th August 2003) said:

* A family doctor who installed a camera secretly to film a woman using his bathroom ...

which is unclear: was the installation secret or is the sentence a clumsy attempt to avoid a split infinitive?

Some other sentences where splitting is unavoidable:

* I have to refuse to promise to help to write it.

If you want to refuse to definitely promise, how could you do it without splitting and without ambiguity?

* As soon as I read the book I decided to immediately agree to film it if she asked me.

In this case immediately splits to agree, but where else could it sensibly be put?

Sometimes people avoid splitting a non-infinitive: to be greatly regretted is not a split infinitive (the verb is "to be"), but people feel the need to write greatly to be regretted.

Why is splitting an infinitive worse than splitting any other tense?  Why do people who would never dream of splitting an infinitive happily split the future tense, as in I will probably be going?

source: http://www.eng-lang.co.uk/fowler.htm