401K and IRA Contributions, Limits, and Deductions

Last year I worked for 8 months for an employer with a 401K and then worked for 4 months for another employer that did not offer one.  As a result, my 401K contributions are not maximized.  Is there a way I can make additional deductible contributions to a retirement plan, either my first employer's 401K, or an additional IRA, etc.?  If so, how do I figure the contribution limits?