Most problems with cron jobs stem from the difference in environment between you as user in the shell and the environment the cron job is running in. More specifically, when you're logged in the system knows what you mean when you say 'mysql', but when cron runs the job it doesn't, because there's a different path in cron's environment that doesn't contain the path the 'mysql' binary is in.
The easiest solution is to supply the full path to the mysql binary, which you can find either through
whereis mysql or
which mysql (on the command line).
Also, I'm pretty sure you should not type a space between -p and your password, i.e., it should be -pmypassword instead of -p mypassword (the -u myusername is fine though, in case you're wondering).