The 'tu' imperative of -ER verbs and those conjugated like -ER verbs (i.e. 'tu' form ending in -es) is the present tense minus the final 's', e.g. tu manges -> mange - Read more: http://french.about.com/od/grammar/a/imperative_2.htm
The -s is only dropped from the second person singular of -er type verbs the other verbs just use the second person singular or plural , and the nous form without the pronoun. Just watch out though there are some irregular verbs eg être -sec pers plural= étes, imperative =soyez.