I have been meaning to write a piece on this for awhile but it's difficult to have a comprehensive guide on strategy for Koi Koi, as the game is pretty much 50% skill and 50% luck! Here are a few notes from my experience though...(I hope it makes sense)...
Let's start with 'Skill'.
This is broken into 4 elements that a player needs to consider and analyse.
1) The cards in his/her hand
2) The cards in the field of play
3) The cards he/she has captured and needs to make scoring hands
4) The cards that his/her opponent has captured and needs to make scoring hands
Generally, a 'good' starting hand would be 8 cards of different suits as you'll be pretty much guaranteed to make a matching pair.
A bad hand would contain a set of triplets and a very bad hand would contain two sets of triplets. Two or three pairs is a pretty poor hand too.
The things that a player cannot control are the cards that his/her opponent holds, how he/she plays them and finally, the order of the cards in the stock pile. This is where 'luck' comes into play.
'Luck'.
This starts from the moment the cards are shuffled and dealt. You can evaluate the cards you have with those in the field of play and of any that have been captured but as said previously, you cannot control how your opponent plays or what cards are drawn each turn.
Due to this 'luck' factor it is always best to make a scoring hand as quickly as possible even if it is a low scoring hand, as this way you win the round (unless you Koi Koi) and stay as (or become) Dealer, giving you the advantage of going first in the next round. Chasing big scoring hands can be hazardous!
Bullet points...
Try to stop your opponent from scoring. This sounds obvious but sometimes a player can be focused only on his/her own cards that he/she has captured and needs, completely forgetting about the cards their opponent has captured/requires. An example is to make sure that you capture a poetry scroll card and a blue scroll card. Another example is to capture at least one of the cards that make the Boar-Deer-Butterflies hand. Capturing 'Lights' is a given. This way you reduce the number of scoring hands that your opponent can make, restricting them to mostly low scoring hands.
If you are unable to match a suit with a card from your hand with one in the field of play, then you need to place a card on the table that will do the least amount of damage to you. This usually means playing a junk card as you don't want to be laying down a high value card if you can help it. The best junk cards to play are those of the April and May suits as none of their higher value cards are used to make big scoring hands. The times that it is generally acceptable to play a high value card you can't pair are a) if it is the last card in your hand and b) if you hold another card of the same suit in your hand and the other two cards of that suit have already been captured.
Sometimes when your cards are running out you will have to try and play for the draw/tie (unless the Dealer's Privilege rule is in effect and your opponent is the Dealer). You could find yourself in a situation where you are nowhere near making any sort of hand (whilst your opponent is), so your best bet is to frustrate your opponent by evaluating what cards they have captured and then by trying to capture the cards that they need.
* capturing all poetry scroll cards and all blue scroll cards counts as a scoring hand in its own right.
Card Hierarchy
Tier 1
Crane, Curtain*, Harvest Moon*, Phoenix, Sake Cup*
* these being 'first among equals'.
Tier 2
Poetry Scrolls, Blue Scrolls, Boar, Deer, Butterflies
Tier 3
All other Scroll and Animal cards
Tier 4
Rain man
This may seem controversial as it is a 'Light' card but you can only make two of the higher scoring hands with it.
Tier 5
Junk
PS If I think of anything else, I'll add to it.