What makes a strong personal statement?
Personal statements are very subjective and therefore there are few blanket rules that are appropriate to list. Most law schools will prompt you with what they would like to see you talk about, so be sure to stick as closely as possible to those instructions. Outside of that, personal statements, generally, should tell your story to the admissions committee, over and above what is available to them through your academic transcript, LSAT score, and sketch. Be sure to include facts that would highlight what you can bring to the admitted class and underline your ability to succeed in law school. You should avoid restating information that's available on your sketch unless you feel it should be particularly highlighted; if so, do make sure you elaborate on it beyond just bringing it up in a sentence (if all you intend to write is a sentence on it, perhaps it's better suited for your sketch alone). You shouldn't generally talk about the law school itself, e.g., how great it is or how you look forward to going there, as it doesn't bring anything of substance to your statement and takes up valuable character count. However, if a professor or clinic is particularly important to you, feel free to mention it and elaborate a bit on why. It's also best to ensure that your statement is well written, clear, concise, and contains no mistakes (particularly, glaring ones like talking about the wrong school). Finally, you should have someone proof read it before you submit it.
What can I say on my personal statement to offset bad grades or LSAT?
Generally, there is very little that can be said in a personal statement that will offset bad grades or test scores. Unless you had a significant life event (illness, death, or other exceptional circumstances) cause your poor performance, it is unlikely that the law schools would be able to use your personal statement as a way to compensate for low stats. If it is the case that a significant life event has hindered your grades or scores, you should think about applying under the access category (or equivalent at a particular school) so that your factors receive special consideration. If you do not fit into the latter situation, it is best not to highlight poor performance, particularly if you have no reasonable explanation for it. However, sometimes, context does afford some benefit, so you should weigh carefully whether you want to bring up your grades or test scores, and what you will say when you do.
Some schools say that they have "holistic" admissions. What does that mean?
Holistic admissions means that a school does not solely look at your grades or LSAT to determine whether you are admitted. It does not mean that a school ignores or subsantially reduces the importance of grades or LSAT when deciding on applications. It does mean that having extraordinary experiences, background, education, or other "soft" factors may push a mediocre application into the pile of stronger applications. This, in itself, does not guarantee admission either, just a "better chance" of getting admitted. Holistic admissions does not mean that low stats can be offset by extra-curriculars or a well-written personal statement.