What is the best way to prepare for a job interview in a start up company?

Know their competitors. Know as much about your field as possible. Have some examples of your skills ready in your head.
Good luck.

Calm down, breath. Now, although it’s a new company, research it and the major leaders. Also, make sure you know all the latest trends
within your expertise. Read!

Make sure you are dressed appropriately; nails are clean,
shoes are shined, breath is fresh.

Lastly, be YOU. They are looking for technical fit and
company fit. Be sincere, confident and professional.

Good luck!!

Do a little research into the market you are going into. In terms of challenges a new company might meet up with. Highlight your abilities to meet challenges. Emphasis your innovation and any new ideas you have brought to the industry in the past. You could give examples. If you are aware of new cutting edge technologies mention them in passing.

Smile, always ask to be seated before sitting and always thank the person for their time.

It’s always a great compliment to be involved in a start up company so perhaps say that.

Look into what their product line is and think of software development things to say about the line.

Look at the job description and the skill set they want and remember to mention or demonstrate that you have those skills.

Be confident. Be at ease. Think about the interview as just talking to another engineer.

Know how to reverse a linked list off the top of your head. I’ve been asked this question at all my software engineer interviews.

Make damn sure you’re able to code as well as you say you can. When I interview new engineers I make sure they understand basic computer science concepts like data structures, algorithms, linked lists like @timtrueman said. Of course we are not a startup but transactions handle lots of money. Nothing worse than an interviewee that can’t deliver what’s on his resume.