Both "savar shid" and "savar beshid" are correct. However, we use "savar shid" more often as imperative form.

The imperative can be made either with or without the prefix "be-/بِـ", and while both are correct grammatically, one might sound more natural depending on the verb. "Shid/شید" or "besid/بشوید" are informal forms. The formal forms are "shavid/شوید" or "beshavid/بشوید".

Both can be used, but "savar beshid" make the sentence more imperative and "savar shid" make it more polite and less imperative. As AyaH said we often use "savar shid". Also we might say "Lotfan savar shid" or "khahesh mikonam savar shid" which make it more polite.

I think "savar beshid" is more polite than "savar shid". However, as Mohsen said, "Lotfan savar shid" or "khahesh mikonam savar shid" are more polite forms to say "Get on".