In an interview, don't slouch, but don't sit on your hands or tap your foot or anything. Just seem relaxed and confident, whether you feel that way or not. Look up facts about the company or the type of job before you go to it. Make sure to slip any company information in somehow, even if you just have to ask about it.
I once talked myself into a debt collectors job. In fact, that was my second jopb and I'd been unemployed for 9 months before it. I didn't really want to do the work, but I thought I'd give it a go and it was a lot of money. I was never, EVER going to make a good debt collector, but I just wanted some money coming in until they fired me or I left. I don't overly like confrontation, I'm certainly not an aggressive person, and I don't like making people's bad situations even worst (which is what you often have to do, most people in debt are just normal people struggling to get by). Hell, I don't even look like I could be a debt collector, I look like I'm 16.
So how did I get the job? By acting. I can't maintain it throughout the job, but for two half an hour interviews, I can come off as confident, aggressive (don't be aggressive going after most jobs, this was just specific to something like debt collecting), and direct. I spoke firmly and assuredly, I had answers prepared, I'd researched the job and found out what employers wanted in a debt collector, I looked the guy in the eye the entire time I spoke to him, I shook his hand firmly (and held out my hand first)...and so on. Also, he was reluctant to give me the job, given my lack of age and experience, but he said he was tempted, so I seized that opportunity to tell him I understood his concerns, but I KNEW I could do it.
Like I say, I got it. I got fired pretty quickly (I hated it so much i would have quit soon so i didn't care), but that was ok. It was a ridiculous company, the number of clients I had to lie to because their file wasn't even in the filing cabinets anymore and we had no idea who we had to chase down...it was awful. Plus the two experienced debt collectors who were supposed to help me left two days after I got there (one was ill, the other quit), so I never had a hope.
Now I'm not saying you should go for jobs you know you can't do. But even going for jobs you think you could do, the same principles apply.
) Other candidates there had bloody folders and didn't get a job.