The biggest mistake I have seen consistently watching our casual spades server is this: Players do not bid enough.
I suspect this is due to a combination of several factors. First, inexperienced players are often not confident in their ability to take tricks. Second, the hand evaluation is often weak. Third, players are over-concerned with getting bags during the play.
This seems reasonable on the surface, as the risk/reward for overbidding by one trick is not good. Consider bidding six or seven tricks, and either making or going down one trick.
+60 + 61
-70 + 70
One stands to gain nine points and lose 130 points. The expected value is negative, unless the probability of winning the trick successfully is close to one. Since this probability is estimated as a matter of judgment, bidding is only as good as one's judgment.
As a rule of thumb, I recommend evaluating the hand as follows:
Aces = 1
Kings = 1
KQ(xx) = 1
Every spade after the 4th
If you're the 1st or 2nd person to bid, I add one trick on top of this.