As an addendum to his essay on bitachon and hishtadlut, Rav Dessler gives us several practical suggestions for maintaining a healthy balance between the two. Again, this text is from Michtav M'Eliyahu, Vol. 1 (Bitachon v'Hishtadlut)*:

We explained above that one can minimize endeavor only to a certain extent. The proviso is that one is strong enough to withstand the tests that may result without feeling sorry that one started on the path of faith in the first place. We should therefore look for ways to lessen the risk by strengthening our ability to meet challenges of this sort. Here are some suggestions.

1. Train yourself to be satisfied with less in the way of worldly goods. The less you need, the easier it will be for you to cut down on physical endeavor and the less dangerous will be the test. As our Sages said (Avot 6:4) "The way of Torah is to eat dry bread and salt and drink water by the measure..."   
2. Pray with devotion until you recognize that everything comes from HaShem. Then even if you don't succeed in your endeavors you will know that your lack of success, too, is decreed by Heaven. The deeper you realization of this truth, the less likely you will be to regret the course you have adopted. 
3. Reflect constantly on the importance of trust in HaShem. Read about trust in our holy sources. Even when you are engaged in necessary physical endeavor be aware that you are only fulfilling the decree "By the sweat of your brow, etc." (Gen. 3:19) Realize that what you will have is decreed and the extent of your endeavor will not alter this. All this will serve to strengthen your faith. Be prepared to close your ears to the taunts of those who call you "good for nothing" and similar epithets. Better to be called this and worse, than to abandon one's principles. Remember the words of our Sages (Eduyot 5:6): "Better I should be called a fool all my life than be a rasha before God for one hour."
4. When a person has to do something which he feels is unworthy of him he is affected by a sense of shame and is impelled to do it in private. This is an instinct implanted by HaShem, and dates from after the sin of Adam. before that the Torah says "...and they were not ashamed." (Gen 2:25) At that time physical desire was not within their experience at all. But afterwards, since it became impossible for a human being to achieve complete unselfishness, he was endowed with a sense of modesty which taught him to be ashamed of selfish physical desire. Similarly with economic endeavor, even thought the person may have no choice (after all, it was Adam's sin that brought this upon us), he should still engage in it with as much modesty as possible. He should experience a sense of shame that he has to do actions which seem to contradict the all-pervading power of HaShem. Just as modesty is effective in limiting physical desire, so it is with physical endeavor; modesty and restraint can serve to keep it within proper bounds. There is an interesting comment by the Vilna Gaon on the word "cheresh" used in connection with the sending of the two spies by Yehoshua. He writes: "This means that he sent them in silence...so that they should not fail as the earlier spies did." This provides remarkable confirmation of what we said above [in the essay].
5. "Make your Torah fixed and your work casual." (Avot 1:15). To achieve this one has to make Torah one's main ambition. One must strengthen one's determination to achieve success in Torah to such an extent that one is prepared to give up one's life for it, in accordance with the words of our Sages. When one's life-ambition is for spiritual things this will automatically lessen the urge for physical endeavor and one will also spend less time on material things generally. He will accordingly be less prone to regrets if he is not so successful in the material sphere. Regret is after all only a function of unfulfilled ambition.

 *English adapted from Strive for Truth! by Rabbi Aryeh Carmell