Although there are many different ways to approach the legislature to advocate for a particular position, there is one item that is at the core of any approach - always tell the truth. There is only one opportunity to establish credibility, so it is never a good idea to exaggerate when talking to legislators.
Legislators are aware that your version is likely only one of many opinions on any given issue; if you are aware that other opinions exist, share them. Not only does this demonstrate that you have a full understanding of the issue at hand, it also reiterates that you are being honest and not trying to hide anything.
If you are asked a question that you do not know the answer to, never guess. It's better to admit that you don't know and commit to finding out. Always be sure to follow up with that legislator at a later date with the correct information.
The following resources will help you better understand the legislature: General Tips for Effective Advocacy (PDF)