Whether you follow a Agile framework like Scrum with its planning poker or a traditional project management approach, the estimation activity is always difficult to perform productively and consistently on the long term.
In this article, Kirill Klimov shares a simple and lightweight estimation technique that should make the sizing process easier for a Scrum team. Story point is a relative sizing metric, which means that you always compare stories between them. This often works well at the very beginning the project, but when you estimate new stories, it becomes much difficult because the calibration of one story point is forgotten.
Kirill Klimov suggests to create an “estimation net”, which is a visual table with one row for each estimation size you use. Having visual references during the sizing process helps the team, because there are real examples within each size and people can compare new stories to these examples.
Read the complete article on http://www.scrumalliance.org/articles/481-the-estimation-net