The second best advice I've ever gotten about how to play blood bowl was (although I cannot remember where I read it to give credit) ...
Move you players as if EVERY dice roll was going to fail and assume everything your opponent will do will succeed.
This should reinforce many good offensive practices such as standing up players who were knocked down first, positioning players before any blocks or actions are taken and minimize the number of dice rolls you will need to complete your turn.
The best advice that has been plastered all over the forums is ...
Read the rule book. I would add "and understand it."
With no prior knowledge of the game mechanics, it took me around 100 games against the AI to figure out what all the rules meant and how I could apply them to how I played the game. I won a majority of the games but also lost many that I could have won if I'd played soundly. This game has a learning curve that everyone needs to go through and it will take time.
One last bit of wisdom is that this game brutally punishes the risk-takers. It only takes one failure to end your turn. You will find it extremely advantageous to do the least risky action then then next risky and so on until you have completed your turn.
Good luck and enjoy the mayhem