Unless you think declarer is going to run out of trumps — and that is not an every day occurrence — there is little point in trying to establish a long suit. Remember that once declarer or dummy is void of a suit then that hand will be able to ruff your winning cards. In general terms your aim should be to set up some fast winners — the tortoise will definitely not beat the hare in this contest! Probably everyone knows this table:
From KQJ5 lead the king
From KQ105 lead the king
From KJ105 lead the jack
From QJ105 lead the queen
From QJ95 lead the queen
From J1095 lead the jack
From J1085 lead the jack
From 10985 lead the ten
However in these two examples: AK1065 & AK865
The situation compared to no-trumps, changes. At no-trumps the recommended lead was the six. Against a suit contract that would be very silly as you could easily allow the declarer to score a trick, perhaps even with a singleton queen. The correct strategy is to start with the king, giving you a chance to look at the dummy and formulate a plan based on what you can see in the dummy. Add that to the information you will have from the bidding and partner’s play to the first trick and you are almost certain to off to a good start.
It won’t always be so simple!
The opening lead is the one edge you have over declarer. The subject is so complex that several books have been devoted to this single topic. When it comes to a suit contract I have two pieces of general advice to offer. Opposite a silent partner you will only have the opponent’s bidding to act as a guide and that may not tell you much. If you have an awkward choice then never underlead an ace and think twice before you lead away from a king. Faced with a choice between these two holdings: 
Q 7 4 & 
K 7 4
I would go for the four of hearts. Neither lead is completely safe but leading from the queen is less likely to be a disaster.
If you are leading from a four-card suit that does not contain a sequence then fourth highest is still correct. So from suits like K872 or Q962 the two would be the card to lead.
When there is a trump suit it can be attractive to lead from a short suit, as you may be able to score a ruff. My advice is that leading a singleton or doubleton can work well and it helps if you have a degree of control in the trump suit. This is what I mean: 
If the opponents are in a spade contract a heart lead may enable you to collect one or more ruffs, because your ace of spades prevents all your trumps from being drawn immediately.
It has become an established custom to avoid a short suit lead when you have a good holding in the trump suit. The idea is to start with your longest side suit in the hope that repeated leads in it will make the declarer use up too many of his trumps. The so-called forcing defence.
This would be a typical hand: 
If hearts are trumps the suggestion is that you should lead the four of clubs. I have never been entirely convinced that this is correct! Because declarer almost always has more trumps at his disposal than your side you may have to wait a long time before you find a hand where this is better than going for the simple shot of leading your singleton diamond.
I thought this was pretty heretical stuff but just as I was finishing this little piece an article entitled ‘The Macho Lead’ by Swedish International Anders Wirgren supporting this theory appeared in Bridge Today.
When you lead a singleton your partner may have the ace and be able to give you a ruff at once. That is less likely to happen if you start with a doubleton as your partner will need more in the way of high cards. Nevertheless it can still be a good idea to start with such a lead. The normal lead from two cards is the highest one so from 
105 you would lead the ten. You will sometimes be tempted to lead from a doubleton honour, say, A 4, K 4, Q 4, J 4 or the like.
Oscar Wilde once remarked that he could ‘resist everything except temptation’ but my advice is to try and avoid starting your defensive campaign with a lead from one of these combinations. You may score the odd triumph when partner has a decent holding in the suit, particularly when you have the ace or king but more often than not you will concede a tempo and maybe a trick.
The situation changes if you partner has bid a suit. Now it is usually going to be right to start by leading it. With a doubleton you start with you highest card. With three or four cards in the suit you should lead your lowest unless you have the ace or two touching honours. Holding say A 7 5 or A 7 5 3 with your partner having bid this suit the right card to lead would be the ace. With a lesser honour it becomes correct to lead a small card. This example shows you why:
If you start with the king of clubs it will cost your side a trick. By leading the four of clubs you allow your partner to win with the ace and return the jack, rounding up the queen.
I broach the next topic with a degree of trepidation — when should you lead a trump?
The reason is simply that the risk factor is very high. You are sacrificing a tempo by starting on declarer’s best suit rather than one of your own. The main reason for leading a trump is in an attempt to prevent the declarer from making a lot of trump tricks separately. This will usually be when you have a powerful holding in another suit bid by the declarer. Imagine that the declarer has bid hearts and diamonds and ends up in hearts. If your hand is: 
Then by leading a trump — the two is the right card from this type of holding — you may prevent declarer from ruffing losing diamonds in the dummy.
If your hand is so bad that you face a choice of leads from poor suits such as 9753 or 963 then you are in trouble! Whatever you lead partner is likely to be fooled. A high card will look like it is from a doubleton whilst a low one will make partner think you have an honour. My personal preference is to lead the second highest from four cards and the middle one from three. This latter rejoices in the wonderful name of ‘MUD’ standing for ‘middle, up, down’ the idea being to play the higher card on the second round of the suit to let partner know you didn’t start with a doubleton.
If your opponents bid a slam against you then your priorities change a little, as time may not be on your side. Like so many areas of the game the subject is complex and there are lots of ideas on the subject. One distinguished magazine devoted an entire series of articles to the question of whether or not you should lead an ace against a slam contract. I don’t recall the conclusion but it is certainly a double-edged sword as this hand illustrates:
North-South bid simply and aggressively to a slam via the sequence One Spade-Four SpadesSix Spades. With nothing to go on West led the ace of diamonds — the only card to allow the slam to make. Change the North-South hands to:
and anything other than a diamond lead allows declarer to get rid of a loser on dummy’s king of clubs. If you have an ace your choice will usually be between leading it or trying to establish a trick in another suit, hoping to get in with the ace to cash it.