For a given development strategy, how do you ensure enough secondary resources?
There is no sure fire path to victory in Settlers, since there will be some games where a six is never rolled and a twelve is rolled four times in ten turns.  Nevertheless there are some principles to follow when playing Catan.


Watch the resources you don't have. Know who is holding them and who is not holding them.  Often times the entire board will be short on a resource and at other times there will be an abundance of something you don't have.  Look for trades when there's lots out there.  Look to soldier when there isn't.  Managing your acquisition of your most limited resources is the most important skill in Settlers.
Don't waste soldiers.  Hold your soldiers and only use them when they can provide you with a big advantage.  If you have no need for the resource that the thief is sitting on then don't worry about it unless it's worth multiples for you.  Unplayed soldiers are valuable bargaining material. You may be able to get a good trade by threatening others.  Use your judgment though, some players won't take kindly to threats. Ignore this rule if another player may snipe largest army. 
Try and create jackpots.  Having a jackpot number makes you less of a target when you don't have your number come up and allows you do almost always do something when your number does come up.
4-1 trades can be worth it, but only do so after exhausting diplomatic/military options.
Never hold the resources which are scarce to you.  It's better to burn the sheep on dev cards, the brick and wood on roads than to try and save them for another round to get a settlement.  Inevitably, if you risk holding them, you will be thieved or lose some other form of opportunity.


You will be short of resources in every game of Settlers.  You can't have good incomes of all the five resource types, so it's important to understand your current game situation and how to maximise your advantages, while minimising your weaknesses.  Development cards are excellent for papering over the gaps in your civilisation.  Brick/wood players are trying to out-expand their opponents.  Wheat/ore players are trying to advance their civilisations so quickly that they can out-produce their opponents.