This is no easy question without knowing the exact context or situation in which to use either of the two expressions.
So, I will begin with the part of your question that I could answer with complete confidence. No, 「楽しみにしとく」 does not mean "I enjoy myself in advance about it." regardless of the context or situation.
Both 「楽しみにしている」 and 「
楽しみにしておく」 are used to talk about possible future events, period.
Is it happening to you directly?
May the possible fun/happy event happen to you directly or to another person in your social circle? If the latter is the case, how close are you (the speaker) to that person?
「楽しみにしている」 is likely be the phrase choice for you if you are that person himself or someone very close to that person. The possible fun/happy event will influence your life more than it will others. 「しておく」 would be a highly unnatural choice in this case.
「楽しみにしておく」 might well be the phrase choice if you are not very close to that lucky-person-to-be. Of course, you might still choose to use 「している」 to show your interest more.
When might the fun/happy event occur? (How far in the future?)
Regardless of the amount of direct influence of the possible event, you might actually choose to say 「楽しみにしておきます」 if the event will not occur anytime soon in the first place.
It takes more energy to be 「楽しみにしている」 than to be 「楽しみにしておく」 because the former is something one does more actively than the latter. 「楽しみにしておく」 could even mean or imply "to forget about it now and start thinking about it when the day approaches".
In conclusion, the phrase choice is influenced by multiple factors. There is no absolute rule as to which to choose when. This is why I said it was no easy question at the beginning. You do not want to sound either unusually interested or overly indifferent.
When in doubt, you will always have a safe alternative of just saying 「楽しみですね！」 with a smile.