I used to ally myself with the Pope all the time but now I sort of consider it cheating. Plus it doesn't make any sense when you get excommunicated yet are still an ally.
Scotland are notoriously hard to get excommunicated. So long as you have a well defended border they will never attack, yet the moment you attack them the Pope seems to hate you and you can bet that no matter how much Papal favour you have it won't be long before you're threatened with excommunication. Therefore it's best to blitz Scotland at the start of the campaign, when excommunication is less likely to have a major impact. Even when I've been excommunicated, the first crusade has always been against either Antioch or Jerusalem, giving you time to get reconciled with his holiness before the next crusade can be called. Also, on the off chance a crusade is called against you, unlike later on, not many countries are going to be in a position to send wave after wave of full stack armies against you at the start of the campaign.
Your other main enemy, however, the dastardly frog eating French (no offence intended), should be less of a problem to get excommunicated. You could try and bait them by leaving Caen lightly garrisoned, or if you don't mind waiting until later on then no doubt they'll get themselves excommunicated fighting another faction. Then, so long as the Pope likes you, you should be able to use your influence to call a crusade and being centrally placed in Europe, as opposed to Scotland who are pretty isloated, a crusade against France should attract a lot of attention from their neighbours.
[This message has been edited by Brutus93 (edited 07-24-2011 @ 07:40 AM).]