Holding Q J - on your first lead when the Queen is led, it shows a singleton or a doubleton with the Jack.
Holding A K doubleton - lead the Ace followed by the King.
Holding A K 6 3 2 - lead the King followed by the Ace, this shows that you have some smaller ones in that suit.
Never under lead your Ace in a Trump contract. This is perfectly fine though in a No-Trump contract if you have length in that suit.
The top of a sequence is always a good lead [K,Q,J] - lead the King, [Q,J,10] - lead the Queen.
If you have a Singleton in a Trump contract, this is also a good lead.
Avoid leading your Ace [A-9-5-3] - the sole exception is if the opponents are in a Slam contract.
Always lead back your partner's suit, presumably they have led from an Honour or a singleton.
Once the dummy hand is down, if this is on the left of you, try to lead a small card through the Strength of the Board - i.e. [A,Q,9,6,2]. If the Dummy is to the right of you, then lead to the weakness of the Board - i.e. [7-5-3].