Espescially in the case of working in London; you have to be careful.
At design agencies I am always on my best behaviour, because I am conscious that I want people to like me, or atleast not dislike me. An internship is kind of like a really long and horrendous interview, and you wouldn't say anything to annoy an interviewer, would you?
A friend of mine took advantage of a design agency and I fear he will regret it greatly. After arranging at short notice and short-term placement, because another agency let him down, he turned up late, had an extended lunch break and after two days announced he wouldn't be there for the rest of the week because he was heading home for a few days; family holiday. The agency had to sack him; they 'employ' alot of people from the same universities (and we all know eachother) so to set an example they had to be firm. My main fear though, is that his name will now be blacklisted among the design world. His name will go around like a dirty chinese whisper, and finding further placements will become hard. My friend has also set a poor example to the agency for students from our course, which is dissappointing.
Ofcourse, small-village gossip can be good for getting placements. Many agencies I have been to are more interested in where you have been, than what you have done, and I am fairly certain they just give their friend at that agency a call to see if you're worth the bother. As long as you work hard and be nice to people, (as championed by Anthony Burrill) you will be fine.
Networking is also very key to cracking this industry and it is not something I am very good at. I have been told that it is a good idea to leave every placement with atleast two contacts. I have another friend who goes around the office in the last week of her placements and gets email addresses from anyone that will give her one. This has enabled her to get into some of the top agencies in London. She has her heart set on getting a placement at one particular agency and I think by doing this, she could very well get it.