There are several components to getting laws changed so that public services can better include homeless people and so that the government can better protect homeless people.
First, it is necessary to develop a very clear statement about how the law needs to change.
Examples:
If government services exclude homeless people because they lack an address or particular ID documents, maybe advocates should petition for exceptions to be added to those particular regulations .
If the crimes code doesn’t adequately deal with violent attackers who target the homeless, then perhaps the state’s hate crimes law can be modified to include crimes against the homeless as hate crimes.
The National Low Income Housing Coalition has an excellent online compilation of its Congressional testimony, letters to lawmakers, and authoritative commentary on proposed laws and regulations. http://nlihc.org/library/testimony/testimonies Peruse these documents to see good examples of concise and persuasive ways to state your concerns.
Here are some samples of legislative issue lists from assorted homeless advocacy groups:
Vermont Coalition to End Homelessness
http://www.helpingtohouse.org/advocacy.php
Rhode Island Coalition for the Homeless
http://www.rihomeless.org/AboutHomelessness/Solutions/LegislationWeAreFightingFor/tabid/267/Default.aspx
Washington State Coalition for the Homeless
http://endhomelessnesswa.org/action/
Minnesota Coalition for the Homeless
http://www.mnhomelesscoalition.org/
Second, advocates need to identify and involve legislators whose committee work directly relates to the problem needing to be solved.
A great example of this is the work done by Sapphire Jule King, Founder and President of the International Freedom Coalition who first declared that Rhode Island needed a Bill of Rights for Homeless People. http://strongfamiliesnow.org/blog/2010/11/26/bill-of-rights-for-the-homeless/ See also Ms. King’s analytical editorial about how that final Bill of Rights failed to include critical protections against abuses by shelters, despite all of the input from shelter abuse victims. Along with John Joyce of the Rhode Island Homeless Advocacy Project and the Rhode Island Coalition for the Homeless, the International Freedom Coalition notified the state’s Human Rights Commission and Senator John Tassoni, Chair of the Rhode Island Senate’s Chair of Housing and Municipal Government, about a range of substandard government treatment of homeless people including the fact that they are often blocked from state government services because they lack street addresses to put on forms and the ways they are harmed by homeless sweeps and other arbitrary government seizures of their possessions. They even got the senator to visit a homeless shelter. http://www.strongfamiliesnow.org/documents/Homeless-Study-Working-Paper.pdf
http://strongfamiliesnow.org/bor/bill-of-rights-for-the-homeless/
Third, collect and share with legislators and the public reliable information to raise awareness about the issue.
The National Coalition for the Homeless does a masterful job of compiling facts about homeless life and has good experience at influencing legislation. Read through some of their handbooks to see how to draft petitions, compose invitation letters to politicians, write a press release, draft a model law for the legislature to build-on, generate community support, etc…
Hate Crimes Manual— a packet of sample documents and instructions for getting your legislature to include attacks against homeless people in the hate crimes law.
http://www.nationalhomeless.org/publications/hatecrimes/hatecrimesmanual10.pdf
Fundamentals to Prevent Homelessness—a packet of documents and instructions for getting political candidates to pledge that they will work to prevent and reduce homelessness.
http://www.nationalhomeless.org/publications/fivefundamentals/index.html
The National Association of Social Workers has a helpful Lobbying Handbook http://www.naswct.org/displaycommon.cfm?an=1&subarticlenbr=19 with practical instructions to help you through the steps of writing to legislators, organizing visits to the legislature, knowing how to testify, and other ways of making sure that your message gets through to lawmakers.
The National Conference of State Legislatures (NCSL), in its Human Services issue section http://www.ncsl.org/issues-research/human-services.aspx?tabs=858,50, provides information to legislators who are trying to develop new laws and amend old laws with provisions for people in poverty. The site specifically says that “NCSL staff can provide [for legislators] comprehensive, thorough, timely and in-depth information on critical human service policy issues.” The same site also tells about and links to recent federal and state legislation on assorted poverty issues.
Homeless Law Blog 
I’m an resident living in a homeless shelter in Henderson, Ky.
The program is a 3-4 months, and we have only a limited amount
of time. I’m new here in the area. I have been living here for about
3 months now. I moved here in November, and obtained a new job
at the 1st of the year. The shelter manager told me that she can
only give me an extra week to stay, when my 3 months stay has been
fulfilled. Just recently, I’ve lost my new job, and the shelter manager
has changed her mind. It was told to me, that I have to Friday to find a job. Basically, I have 3 days to find a job. If I don’t, I have to leave the
shelter. I have no where else to go. I don’t know anybody here, and have
no family. I’m basically back in the same situation again, homeless.
Is there something that the legislature can do to prevent me from
living out on the streets?
I have a clean record; never been on drugs, nor alcoholic, and I feel
like that is a cruel decision to make by the shelter manager.
The shelter manager has pulled strings for other residents who are
court-ordered to be there in the shelter’s program.
She has helped numerous drug-court residents that have a criminal
background, but which I don’t understand.
She and the shelter board voted against another staff worker for
wanting to take me to my section 8 appointment to receive my housing
choice voucher. The result of that was: I lost my housing choice voucher.
The housing division removed my name off of the waiting list.
The point is: they want us to move out in a hurry, but expect us to have
a place to live in about 3 months. We don’t always get what we want
at an specific time.