One should preferably stand while reciting the brachah and counting (Shulchan Aruch 489:1, Mishneh Berurah 489:6, Aruch Hashulchan 489:4). This is alluded to in the Torah [Devarim 16:9] "May'hachol cher'mash b'kammah" (when the sickle is first put), Chazal understood to be read "B'komah (standing)" (Shaar Hatziyon 489:7). Another reason is brought in the Zohar [Tetzaveh 183a] says that sefira is an very important, just like Shemoneh Esrei. Therefore, we stand just like Shemoneh Esrei (Aruch Hashulchan 489:4).
Therefore, one should be careful not to be lean on anything while counting (Koveitz Halachos 6:1). If one did not stand, he has nevertheless fulfilled his mitzvah (Mishneh Berurah 489:6, Aruch Hashulchan 489:4). Someone who finds it difficult to stand [for example, an older person] may l'chatchila count while sitting down (Koveitz Halachos 6:1).
One should not interrupt in between the reciting of the brachah and the counting. This includes even a silent interruption of more than toch k'day dee'bor, or saying something that is not sefira related (Mishneh Berurah 489:29).
One may count in any language, as long as he understands what he is saying. If one counted using a language that he does not understand, he has not fulfilled his obligation, even if he counted in Hebrew (Mishneh Berurah 489:5). Harav Shmuel Kamenetsky, shlit"a, understands that when counting in Hebrew he is not yotzei only in a situation that he had no idea what he was saying. However, if he was aware that he was counting even though he did not understand the exact translation of the words, he has fulfilled his obligation (Koveitz Halachos 6:5). One may not fulfill his obligation of counting with either thinking or writing (Koveitz Halachos 6:3-4).
As mentioned, when the Torah refers to the mitzvah of counting sefira, there is a mention of both days and weeks. Therefore, the mitzvah requires counting both the days and the weeks of the Omer (Gemara Menachos 66a, Shulchan Aruch 489:1 & Mishneh Berurah 489:7). The first 7 days, one only mentions the day. Starting from the 8th day we mention both the day and the week.
If after from the 7th day, one accidently only mentioned the day and not the week [for example, on the 12th day one counted "today is the 12th day of the Omer" and did not say "today is the 12th day, which is 1 week and 5 days of the Omer"] there is a machlokes whether he has fulfilled his obligation. Therefore, one should recount that night without a brachah [and continue counting with a brachah the next night]. If after the 7th day, one accidently just counted the weeks [for example he said "today is 1 week and 5 days of the Omer], he has not fulfilled his obligation, and should recount that night with a brachah [for it is as if he never counted] (Mishneh Berurah 489:7).
If one counted using roshei tay'vos (an acronym) [for example, instead of saying "today is the 12th day etc.", he said "today is yud bais yom etc." there is a machlokes whether he has fulfilled his obligation or not. Therefore, he should recount that night without a brachah [and continue counting with a brachah the following night (Be'ur Halachah 489:1 s.v. moneh v'holaich).