How to write Acknowledgements effectively?

Acknowledgements are usually included at the beginning of a long piece of work such as a dissertation or thesis. 

Example acknowledgements 
This is obviously a very personal part of your work; the extract below will simply give you an idea of the sort of language used. 

I would like to thank the following: Dr. Fred Titmus, my supervisor, for his encouragement, patience and expert advice; Rachel Heyhoe-Flint, for supplying so much invaluable information; members of the Learner Development Team, for help with my English and approaches to writing; and finally special thanks go to my friends and family who have supported me throughout my research.

Other expressions you might use include:

I am (most/very) grateful to ... 
I wish to (warmly) thank the following ... 
This paper could not have been written without the invaluable help of ... 
I acknowledge my gratitude to ... 
You can of course look at books you have used or at copies of previous dissertations/theses.