For writing skill... I think you can also learn from reading many books
After you read a few books, you will start to remember the way they arrange the word
it would also help you with the vocabulary as you have to learn new words all the time
But like Mint said... Practice makes perfect
The more you practice, the more you learn and get more advanced
(but while you practice you'll have to check for errors too)
just my opinion though..
For speaking (informally) ..I think the primary objective is communication
...If that person understand, then that's it. you succeed already
A few weeks ago I went to Paragon. I saw a Farang shopping with his Thai wife
the guys was about to buy an expensive kitchenware that looks like a hand blender
"For chopping the vegetables สับกระเทียม สับพริก ดีไม่?"
and the wife answer...
No no no.. too much price... You, I can do with my ครกอ่ะ ป๊อกๆๆๆๆๆๆ อ่ะ finish! easy!
That's it.... Pure communication...... No accents, no grammar required