An assessment appeal involves a process of monitoring the assessing authority to make sure your assessment is accurate and equitable, there are 3 steps to this process....

Informal Appeal
What You Can Do:
Look at the new market value of your property. Be objective as possible. If you believe the market would not support our estimate of the value of your property, or that your assessment is not equitable with others, please call our office. Be prepared to support your position by offering some evidence of the value you believe to be correct or indicate the value of similar comparable properties.
What The Assessment Office Can Do:
We can explain how we determined your assessed value, quote the values of comparable properties and offer any assistance we can provide. The only issue we can address is the value of your property; the amount of your tax bill is outside of the control of this department. If we cannot resolve this matter, go to the next step.

Formal Appeal Before The County Board of Review
You must fill out the proper form ( available at the Assessor's office in the County Courthouse), you must also submit this form in the proper time frame, again, consult the Assessor's office to determine this.
You will be notified of the dates you may appear and present your case. Your case will be decided on evidence you provide and any evidence the county has or is provided by the assessor - this hearing usually lasts 25-20 minutes.
Helpful evidence to present:
1.   Recent sale price of the property being appealed.
2.   Recent appraisal of the property being appealed.
3.   List of recent sales of comparable properties.
4.   List of similar property assessments.
5.   Photographs
6.   Sketch of property to confirm sizes are correctly recorded.

If the County Board of Review decision is not satisfactory, property owners have 30 days from the date the Bd. of Review's written notice is mailed to appeal to the State Property Tax Appeal Board.

Appeal to the State Property Tax Appeal Board
Only an owner of property or taxpayer dissatisfied with the decision of the County Board of Review may file an appeal with the State Property Tax Appeal Board. Appeals must be filed on the proscribed form within 30 days of the postmark date of the County Bd. of Review decision. Anyone needing assistance may contact the Clark County assessors office to obtain these forms for filing.
The Sate Property Tax Appeal Board maintains a web site with a full description of requirements:   http://www.state.il.us/agency/ptab/

Several Exemptions are available to homeowners:

If the Homeowners own and reside in their property, they are eligible for a General Homestead Exemption of up to $5,000 to be deducted from the assessed value before their tax bill is calculated.

Homeowners who improve an existing property (an addition, garage, deck, etc.) may qualify for a Home Improvement Exemption which could delay the amount of increase in the assessed value for a four year period.

Persons who are 65 or older are eligible for a Senior Citizen's Exemption, which deducts an additional $3,000 from the assessed value before their tax bill is calculated.

The Senior Citizen Freeze Homestead Exemption applies to those who meet the following qualifications:
1.   Turn 65 or older this calendar year.
2.   Are the owner of record of the residence.
3.   Were living in the residence on January 1 of this year and they apply on        January 1 of the previous year.
4.   The total previous year's income of everyone living in the residence on January 1 of the year they apply is $40,000 to $45,000
This exemption freezes the assessed value - not the taxes. If taxing districts ask for more dollars, your tax bill could still increase.