Kind of a general question, so I'll give a general answer:
Always be learning.
Sure, this advice applies well to ANY software engineer, but as a STARTUP engineer, bleeding edge technologies are your specific realm. Day-jobby cubicle jockeys can chip away at commodity software with commodity tools for years and make solid living, but the startup engineer needs to intimately understand the Next Big Thing(s) and be able to put them to use bringing new innovation to market for cheaper than ever possible before.
Some examples that are relevant today (for the average, blue-sky startup idea that needs to get to market fast and build incrementally):
- Ruby, Python > .Net, Java
- MySQL, PostgreSQL > Oracle, MSSQL
- NoSQL/schemaless DBs > traditional RDBMS
- Cloud Computing > traditional hosting/computing infrastructure
- StackOverflow > MSDN
- DVCSes like Git, Mercurial > CVS, SVN
- Github, Google Code > SourceForge
- Agile, short iterations > Waterfall, 2-year dev cycles
But again, that's just the situation as it stands today. In 5 years (maybe 2 years), the left side of this list will be on the right, and the whole point is to know what the new, yet-to-emerge technologies that supplant them on the left will be.