I would suggest running your ficlet through a spell-check if it's multi-chaptered before you post, that will take care of most spelling/grammar errors, although you need to set the English language to be edited in it to your own country of origin. For example you'd be best to set your spell-checker to US-English if you live in the US, or UK-English if you live in the UK, and so on, depending on where you live in the world.
Secondly when you get someone to beta your ficlet, you really need to ask them which English language they know, since for example US - English uses different spelling for words than UK - English, so you don't want your beta saying your spelling is wrong when they used US - English and you use UK - English.
For example in US - English they tend to drop the 'u' in certain words like 'Labour' becomes 'Labor', and words like 'Aeon' become 'Eon'.
Here's a good site example of US vs UK spellings of some English words.
US vs UK spellings.
Additionally you should really define what you want your beta to do. Do you want them to just read for plotholes, for spelling and grammar errors, or for punctuation? The beta won't catch everything in the chapter or ficlet, they're only human.
Thirdly, I would suggest that you start proofreading your own work before you post too, even if it is a chapter or a drabble. It'll help you to better understand your own writing, find the areas you need help with like spelling, and make you more familiar with your own writing style. I know it can be a pain, but as a writer you won't develop if you're terrified of reading your own stuff. A good excercise in your own patience is to proof your own work before you send it to a beta too, and making a list of things you've noticed to help them figure out what exactly you want them to do.
Remember that a beta is giving up their free-time to help you, so it's only polite to make things a bit easier for them. If they're a good friend, they likely won't mind you bouncing ideas off of their noggins to help you improve and or even to extend your ficlet or chapter to be posted.
I don't use betas myself since I write rarely now, and I proofread extensively myself, so it kind of negates the purpose of using a beta. But for others I applaude the author and beta for being so brave to have their work critiqued by another. It's very courageous.
~ Pyre