I'd say this infographic from Neomobile was almost perfect.
But I wouldn't recommend always placing your links at the end of the tweet, as that's the first bit that'll get cut off if people do an old style retweet or append copy to the start of a reply. It's safer to put your links in the middle of the tweet-even if that may look a bit ugly-so the integrity of the link is never compromised.
After all, a tweet with a broken link is no use to anyone.
So, on balance, I think I'd recommend this sort of structure:
(CTA = call to action)
What do you think?
Here's Neomobile's recommendation for comparison: