It's very annoying when you want to outbid another Rinkya bidder with your own Rinkya account- you may get the item a little cheaper based on the final sale price, but assuming the item sells for under 10,000y, you have to pay a further US$13.00 in commission.
When do I bid and WHAT do I bid? It totally varies on the
type of the auction, the
seller and how much
time I have. [Keropi will love this response because of the analysis

]
Type- Some series are popular and will get bids throughout its duration. In that case I needn't bid early because my initial bid will likely be overridden quickly. The only time this can be of benefit is if I want the auction in my 'Bid History' list for reference purposes.
Other series have a select bunch of fans whos bidding habits I know. In that case I've learnt to wait till the end of the auction before starting the bidding tactics.
By the way, for those who say there's no tactics in bidding, that's not true. Part of the fun is psyching out the opposition and it does play a part in auctions.
If you're bidding on a cel from a relatively competition-free series, bidding early is good. If a potential bidder/opportunist already sees a bid on a cheap cel, they'll probably turn away and leave you to it.
If you were to wait till the dying minutes, that opportunist may already bid, thinking they're getting a good deal. But if you REALLY want the item, you'll then have to pay more to beat that initial bid. Even if it's just 100 or 250yen, those add up over time, especially if it brings you over a deputy's commission tier
Seller- As Keropi mentioned, some sellers will delist the item if there's no bid in the last 24 hours. When you know which seller might this, it's best to get in there early or risk missing out.
Having the Time- On occasions we can't always be there in the dying moments to monitor the state of play. As such it's sometimes necessary to bid early and it's worth bidding a little more than you'd like to pay.
I get annoyed if I'm outbid by a small denomination like 100y, so I'll bid more than my budget allows so if I still loose, I know it was out of my reach and I'm satisfied.
I remind myself that each piece is unique and that if someone gets it, I'll never see it again. That's how I justify spending more on certain artwork.