It goes without saying that due diligence should be performed first, checking ICOs on multiple sources, cross-referencing reviews, join telegram, etc.
1) Most decent ICOs require you to be whitelisted + KYC. If the ICO is trendy, the whitelist could easily close a few days/weeks even before ICO. If it looks good, don`t overthink subscribing.
2) KYC can sometimes be troublesome even if you are uploading a high-quality ID Scan. It´s best to have Id and passport scans handy just in case (on a given ICO yesterday, that performed a KYC during the ICO token sale, I had to fast-change from one document to another because of KYC rejection).
3) Be ready for the ICO token sale ahead of time. Sit at the computer at least 30 min before and check conversations on telegram. Go through ICO token sale instructions provided by ICO.
4) Most ICO token sales are ETH based. Set the MEW wallet (or other) parameters according to ICO needs (normally provided by ICO are Gas and GWEI parameters to set -> see whether setting these higher can effectively give you a better chance or could on the contrary act against you for going over limits). Distinguish between suggested values and limits.
5) Be very wary of where you send you’re ETH to (wallet address). Many scammers are out there and on social networks to lure you to a different address (on a given ICO yesterday, ICO’s website was hacked and apparently ETH personal contribution address was swapped for the hackers ETH address.)
6) Check how ICO green flag is set (given hour, given block number, given ETH address in your login area, etc.).
7) Be swift participating when green flag is down (although it is not infrequent for external factors to go against you at this moment: ICO website sometimes falls, then the ETH network tends to suffer and take a long time to process during heavy ICOs).
Check that your tokens are actually transferred to you. If this is not immediate, keep track of when and how they will be released.ICO procedures vary from one to another, and so does the hype around them. These factors combined have a heavy weight in determining whether you eventually get to participate or get left out.
My experience so far in hyped ICOs is that of getting in in two and bounced back from another two (once due to above mentioned hack on ICO main page, another due to a Gas race with no experience in the field at the time with the parameters).
ICOs that are not extremely hyped allow for a more comfortable participation process.
P.D. I have not participated in ICO pre-sales since they tend to be for bigger contributions, so I cannot report on it.